ABSTRACT A historical cohort study of the morbidity of CS2 and control workers was carried out in a viscose rayon factory and an adjacent cotton mill. A total of 725 male workers was employed at the two factories on 1 May 1966. All records of medical treatment received by these men from general practitioners, hospitals, and plant clinics for the period from 1 May 1966 to 31 December 1970 were obtained from receipts issued by those medical institutions and kept in the health insurance office. The study population was divided into five categories according to the duration and severity of CS2 exposure. Mean ages were found to be almost the same for all five groups (range of means 38·1-39·0 years). Period prevalence rates of cerebral vascular disease, ischaemic heart disease, hypertensive disease, diabetes mellitus, nephritis and nephrosis, mental disorders, and neurological diseases were calculated for all groups. Renal disease and hypertensive disease were significantly more prevalent in the rayon spinners and cutters with exposure histories of 10 years or more, the most highly exposed group, than in the non-CS2 workers. When the non-CS2 workers and the CS2 workers with negligible exposure are combined and taken as a reference group, ischaemic heart disease was significantly more prevalent in the most highly exposed group than in the reference group. Ratios of period prevalence rates between these two groups were 7·6 for nephritis and nephrosis, 2·3 for ischaemic heart disease, and 1·9 for hypertensive disease. The ratios for cerebrovascular disease, diabetes mellitus, and neurological diseases were larger than 1·0, but the differences were not statistically significant.