A study was conducted of the prevalence of silicosis for 266 Thai stone mortar workers, 15 to 59 years old, in three villages in the Northern part of Thailand. The subjects were interviewed by questionnaire to obtain information on occupational history, smoking habits, and respiratory symptoms and given physical examinations that included chest X-rays and pulmonary function testing. Fifty six cases of silicosis were detected, yielding a prevalence rate of 21.1%. By age, the highest prevalence rate, 43.3%, occurred in 50 to 59 year old workers. By sex, the prevalence rates were 21.9% in males and 9.3% in females, respectively. By job category, the highest prevalence rate, 28.7%, occurred in workers making stone pestles. The prevalence rate of workers who worked inside buildings was higher than those working outside, 31.5 versus 17.1%. The silicosis prevalence rate among smokers and nonsmokers was 18.8 and 29.3%, respectively. Workers who used clothing to cover their nose and mouths instead of approved dust masks had a prevalence rate of 24% versus 17.2% in those who did not. The silicotic subjects had high frequencies of symptoms such as chest pain, dyspnea, chest tightness, clubbed fingers, and cyanosis. Approximately 92.8% of the silicotic workers had a restrictive ventilatory defect. The silicotics had a high incidence of p-type and q-type radiographic opacities and evidence of tuberculosis, emphysema, and ill defined diaphragm on the X-ray films. A nested case control study was also conducted among the silicotics and 168 randomly selected nonsilicotic workers who used clothing instead of approved dust masks as comparisons. The cases were more likely to be male and older than the comparisons. Workers who polished mortars had a eight fold risk of developing silicosis compared to those who did not.