We studied the sex of children born to individuals involved in the Yucheng oil disaster, Taiwan, who were exposed to polychlorinated byphenyls (PCBs) after an oil contamination accident in 1979. Men exposed to PCBs before age 20 years had a lower chance of having a baby boy than did age-matched and neighbourhood-matched controls (odds ratio 0.65, 95% CI 0.45-0.93). The male-to-female sex ratio of children born to men exposed to PCBs after age 20 years, however, approached that seen in controls (0.90, 0.59-1.35). We noted no significant difference in the birth ratio of infants born to exposed and unexposed mothers (0.93, 0.77-1.12). Our findings suggest that paternal exposure to PCBs before age 20 years affects the sex of a subsequently born child.