The need for an adjustment factor for race in the interpretation of lung function test results was investigated by comparing spirometric data from 1,399 Caucasian and African American blue collar workers. The subjects were 305 African American men, 328 African American women, 410 Caucasian men, and 356 Caucasian women employed in the food products industry, synthetic textile mills, and electrical equipment industry in North Carolina. None of the subjects had documented occupational pulmonary exposures. Forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV1) were determined for all subjects. FVC and FEV1 values were about 13% less for African Americans than for Caucasians, and the differences were constant with increasing age and height. The difference in FVC and FEV1 values between African Americans and Caucasians may be due to the use of standing height to estimate the size of the thorax which is used to calculate an individual's reference values. The FEV1/FVC percentage was approximately the same for African Americans and Caucasians. When a 15% adjustment factor for race was applied, 11.5% of the African Americans were below the lower limit of normal, while without the adjustment factor, 37.4% were below the lower limit of normal. Between subject variability was found to be much greater within an ethnic group than between ethnic groups. The authors conclude that the application of a single adjustment factor for all individuals is not optimal. More reliance should be placed on the use of the FEV1/FVC percentage and on following workers over time. Reference values should be derived from a population with comparable age, physical characteristics, socioeconomic background and ethnic characteristics.