The relationship between chronic (nonreversing) neuropsychological effects and acute exposure effects was investigated in 77 organophosphate exposed male sheep-dippers. Acute exposure effects were assessed prospectively using a purpose-constructed symptoms questionnaire administered pre-, and 24 h postexposure. Urine was analysed for dialkylphosphate levels to confirm recent exposure. Chronic effects were assessed in a cross-sectional neuropsychological study in the absence of recent exposure using computerised neuropsychological tests, the General Health Questionnaire, and the Subjective Memory Questionnaire. Simple correlation and multiple linear regression analyses (adjusting for confounders) were used to assess relationships between the change in total symptom reporting from baseline to 24 h after exposure and chronic effect outcomes. There was no evidence of any association between reported symptom levels and chronic neuropsychological effects. This suggests that chronic effects of OP exposure appear to occur independently of symptoms that might immediately follow acute OP exposure. This has implications for exposure control: individuals may experience chronic effects without the benefit of earlier warning signs of toxic effects during acute exposures.