Organophosphate-based pesticides are widely used throughout the world. The acute effects of over-exposure to such compounds are well known. Concern has also been expressed that long-term exposure may result in damage to the nervous system. In a cross-sectional study, we compared neuropsychological performance in 146 sheep farmers who were exposed to organophosphates in the course of sheep dipping with 143 non-exposed quarry workers (controls). The farmers performed significantly worse than controls in tests to assess sustained attention and speed of information processing. These effects remained after adjustment for covariates. The farmers also showed greater vulnerability to psychiatric disorder than did the controls as measured by the General Health Questionnaire. There were no observed effects on short-term memory and learning. Repeated exposure to organophosphate-based pesticides appears to be associated with subtle changes in the nervous system. Measures should be taken to reduce exposure to organophosphates as far as possible during agricultural operations.