We conducted a validity study to assess whether information bias from differential misclassification of occupational exposures occurred in a case-referent study on mental retardation and parental occupation. The colleagues of 42 case parents and 38 referent parents were traced and interviewed with a questionnaire similar to that used for the parents. Within each parent-colleague pair, Cohen's kappa coefficient was calculated as a measure of agreement between the reported occupational exposures. Averaged across pairs, the mean kappa coefficient was 0.48 (95% CI = 0.40–0.56) for case pairs and 0.46 (95% CI = 0.38–0.54) for referent pairs, indicating little evidence for differential reporting of exposures between cases and referents. Our results also indicate that colleagues are eligible substitute respondents for information on occupational exposures. (Epidemiology 1990; 1:292–297)