A case-control study was conducted to evaluate risk factors for cortical, nuclear, posterior subcapsular and mixed cataract. The 385 cases and 215 controls (age range 40-75 yrs) included in the study underwent a complete ophthalmological examination and laboratory blood tests, and were interviewed about behavioral variables, environmental exposure and their medical history. Lens opacity was classified using the 'Lens Opacity Classification System II' (LOCS II). On multivariate analysis, the risk factors for cortical cataract were the presence of diabetes for more than five years (OR = 3.7) and increased serum K+ and Na+ levels. A history of surgery under general anesthesia and the use of sedative drugs were associated with reduced risk (OR = 0.4). Posterior subcapsular cataract was associated with the use of steroids (OR = 18.2) and diabetes (OR = 8.1), and nuclear cataract with calcitonin (OR = 5.7) and milk intake (OR = 0.25). Mixed cataract was associated with a history of surgery under general anesthesia (OR = 0.5). Some of these results are consistent with the findings of similar studies performed in different geographical areas, others are not. The results suggest a possible role of electrolyte imbalance in the development of senile cataract.