The study was undertaken to determine if occupational exposure to infrared (IR) radiation increases the risk of developing cataract. The study includes epidemiologic investigations of two groups of workers exposed to IR radiation and two groups of non IR-exposed controls. The first investigation included 208 iron and steel workers and 208 controls. For each of the workers, the lifetime IR-exposure was calculated with the help of occupational interviews and measurements of the IR-exposure to their eyes in their jobs. Eye examinations including a detailed slit lamp examination of the lens were made. Exposed persons and controls were examined randomly. Wedge shaped opacities, a common type of senile lens opacity, were found in 32% of IR-exposed workers 60 years and older and in 12% of controls of the same age. In younger age groups there was no significant difference between exposed and controls regarding the presence of lens opacities. The second investigation included 209 IR-exposed glass workers and 298 controls. Workers over 50 years of age and with at least 20 years of occupational IR-exposure were included. Special care was taken to include retired workers. Exposure determinations and ophthalmological examinations were made in the same way as in the first study. The eye examinations of the glass workers showed that 16% of glass workers over 70 years of age had been operated for cataract compared to 1% of controls of the same age. In neither of the two studies was it possible to detect a dose-effect correlation. The exposure measurements showed that the maximal doses in both the iron and the glass industry are about the same. There are, however, a large number of jobs in the iron and steel industry that give low doses while most jobs in the glass works give high exposures. Cataract was found to be more common in the left than in the right eye. Measurements of the exposure to the two eyes separately also showed that the left eye in some working moments is exposed to higher irradiances than the right. A family history of cataract was found to increase the risk for the development of cataract in glass workers.