A prospective study of workers engaged in the manufacture of tolylene di-isocyanate (TDI) was carried out over nine years. The study was in two parts: (1) assessment of the symptoms and ventilatory function of men working on the plants who did not complain of respiratory symptoms; (2) assessment of the long-term effects in men who were removed from the plants because of respiratory symptoms. Use of the MRC Questionnaire on Respiratory Symptoms in a group of 76 TDI workers showed no significant differences in symptoms compared with 76 control subjects. Annual lung function tests were carried out on 180 asymptomatic men with possible exposure to TDI. The relationship of the forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC) to the height, age, and duration of exposure was examined by linear regression analysis. The equations formulated did not differ significantly from those calculated for non-TDI exposed subjects. Thus exposure to very small concentrations of TDI does not necessarily cause symptoms or a drop in ventilatory capacity. Forty-six TDI workers who had developed symptoms reported more long-term symptoms than a comparable number of controls. The difference was significant at the 1% level. Ventilation test records of 61 TDI workers who had developed symptoms were available since it was possible to use the records of men who had left the company. The lung function of this group was matched against predicted figures obtained from a control group of 608 subjects living in the same area as the exposed group but not exposed to TDI. It was found that the FEV1 of the affected men was on average 267 ml lower than predicted, and the FVC 269 ml lower. These findings are confirmed by examination of the indivdual records of a futher 20 men.