OBJECTIVES: The aim of this study was to investigate the possibility of subtoxic exposure to pesticides causing chromosome aberrations in greenhouse workers. METHODS: In a cross-sectional and prospective study design chromosome aberration frequencies in cultured lymphocytes were examined for 116 greenhouse workers exposed to a complex mixture of almost 50 insecticides, fungicides, and growth regulators and also for 29 nonsmoking, nonpesticide-exposed referents. RESULTS: The preseason frequencies of chromosome aberrations were slightly but not statistically significantly elevated for the greenhouse workers when they were compared with the referents. After a summer season of pesticide spraying in the greenhouses, the total frequencies of cells with chromosome aberrations were significantly higher than in the preseason samples (P=0.02) and also higher than for the referents (P=0.05). This finding was especially due to an increased number of cells with chromatid gaps between the first and second samples (P=0.001). The results may reflect an additive genotoxic effect of the spraying season, for which the use of insecticides and growth regulators (but not fungicides) culminates. The highest elevation in the risk of chromatid gaps was observed for persons who did not use gloves during re-entry activities such as nipping, cutting, pricking, and potting (risk ratio 2.88, 95% confidence interval 1.63-5.11). CONCLUSIONS: The present results suggest a genotoxic effect from a complex subtoxic occupational pesticide exposure. In general, the findings indicate the importance of personal protection, during high-exposure re-entry activities, in preventing pesticide uptake and genetic damage.