This study investigated the association between occupational pesticide exposure and semen quality among Chinese workers. Male workers, 32 who were exposed to organophosphate pesticides and 43 who were not exposed were recruited from two nearby factories and interviewed. Following a work shift, semen and urine samples were collected for pesticide metabolite analysis. Semen samples were analyzed for sperm concentration, percentage of motility, and percentage of normal structure. Within the exposed group, the mean end-of-shift urinary p-nitrophenol levels were 0.22 and 0.15 mg/L for the high- and low-exposure subgroups, respectively. Linear regression analysis of individual semen parameters revealed a significant reduction of sperm concentration (35.9 x 10(6) vs 62.8 x 10(6), p < 0.01) and percentage of motility (47% vs 57%, p = 0.03) but not percentage of sperm with normal structure (57% vs 61%, p = 0.13). Multivariate modeling showed a significant overall shift in the mean semen parameter. Occupational exposure to ethylparathion and methamidophos seems to have a moderately adverse effect on semen quality.