On February 16, 1999, the National Institute for Occupational Safety and Health (NIOSH) received a request for a health hazard evaluation (HHE) from the management of the Indian Health Service (IHS) to estimate the prevalence of and evaluate the risk factors for musculoskeletal disorders among IHS dental staff. The present evaluation is a questionnaire survey of all clinical dental employees of the IHS, based on a previous ergonomic study of six Phoenix area IHS dental clinics, as reported in a previous HHE. A list of employees was given to NIOSH investigators by the IHS, and each employee was sent a questionnaire in the mail. Seven hundred and eighty five questionnaires were mailed to the IHS dental employees in the first mailing, and another (identical) questionnaire was sent to those employees who did not return it. A total of 539 employees (69%) returned the questionnaire. Participants were employed in all aspects of dentistry including dental assistants, expanded duty dental assistants, dental hygienists and dentists. Because of substantial differences in job duties between the dentists and the assistants/hygienists, separate analyses of risk factors were done for the two groups. There were 192 dentists and 338 dental assistants/hygienists (9 did not answer the question, and were excluded from the analysis that determined occupational risk factors); most of the dental assistants/hygienists were female (99%) and most of the dentists were male (79%). We used a definition of a work-related musculoskeletal disorder (WRMD) for the five body areas studied (hand/wrist, neck, shoulder, back, elbow) that was previously used in other NIOSH HHEs. A WRMD was considered present if any discomfort (e.g. pain, numbness, tingling, aching, stiffness, or burning) in the affected body part occurred within the last 12 months and all of the following applied: (1) Discomfort began after starting work at the current location, (2) Discomfort lasted for more than one week or occurred at least once a month within the past year, (3) Discomfort was reported as "moderate" (the midpoint) or worse on a five-point intensity scale, (4) Discomfort in the past year was not related to an accident or sudden injury. The prevalence of WRMDs was greater for each body part for dental assistants/hygienists (range, 9-34%) as compared to dentists (range 4-13%). For the dental assistants/hygienists, multivariable statistical models were used to simultaneously assess the relationship between multiple occupational risk factors, confounders, and WRMDs. For dentists, only univariate models could be used, looking at one risk factor at a time. Occupational risk factors were different for each body area studied, and for whether the respondent was a dentist or dental assistant/hygienist. For dentists, neck WRMD was statistically significantly related to not always having a direct view of the patient's mouth (Odds Ratio [OR] 2.1, 95%; confidence interval [CI]1.1,4.1) and fair or poor dental chair comfort (OR 4.5; 95% CI 1.4,14.2). Increased reporting of hand WRMD by dentists was based on extracting 10 more teeth per week (OR 1.4; 95% CI 1.1,1.9) and rating the lighting as fair or poor (OR 6.3; 95% CI 1.0, 20.3). The risk of back WRMD for dentists was statistically related to fair or poor dental chair comfort (OR 3.8; 95% CI 1.0, 14.7) and sitting in the 9 or 10 o'clock position as opposed to the 11 or 12 o'clock position relative to the patient (OR 7.5; 95% CI 1.5, 11.4). Shoulder WRMD for dentists was related to not always having a direct view of the patient's mouth (OR 2.0; 95% CI 1.0,4.0) and the time working at the same location, based on either spending 5-9 years at the same location (OR 4.4; 95% CI 1.3, 15.9) or spending more than 10 years at the same location (OR 7.3; 95% CI 1.6,32.0). For dental assistants/hygienists, neck WRMD was related to not having a fiber-optic handpiece (OR 2.4; 95% CI 1.0, 5.8), fair or poor dental chair comfort (OR 2.0; 95% CI 1.0, 3.9), and the years spent working at the same location (OR 1.3 [for a 5 year increase]; 95% CI 1.1, 1.7). The risk of neck WRMDs in dental assistants/hygienists decreased with an increase in the number of patients per day (OR 0.7 [for seeing 5 more patients a day]; 95% CI 0.4, 0.96). For dental assistants/hygienists, hand WRMD was related to spending more years working at the same location, (OR 1.3 [based on 5 year intervals]; 95% CI 1.1, 1.6). For dental assistants/hygienists, increased back WRMD was statistically significantly associated with locating the handpiece behind the patient rather than locating the handpiece in front of the patient (OR 3.7; 95% CI 1.2,14.2) and spending more years working at the same location, (OR 1.5 [based on 5 year intervals]; 95% C 1.2,1.9). Shoulder WRMD for dental assistants/hygienists was related to having an instrument tray on the left side of the patient versus in front of the patient (OR 8.3; 95% CI 1.3,165.0). NIOSH investigators concluded that some working conditions in IHS dental clinics posed a risk for WRMDs. Dental assistants/hygienists had a higher prevalence of symptom-defined WRMD than dentists. Risk factors for WRMDs included the comfort of the employee's chair, handpiece location, instrument tray location, lighting, the number of teeth extracted per week, not always having a direct view into the patient's mouth, fiber-optic use, the position of the patient relative to the dentist, and the number of years spent at the current location. Changes that the Indian Health Service should make to prevent these disorders are given in the Recommendations section.