A study of semen quality in workers employed at a lead smelter was conducted. The cohort consisted of 119 men, mean age 39.7 years (yr), employed at the Cominco smelter in Trail, British Columbia for whom data on past blood lead (7439921) concentrations existed. Blood samples were analyzed for lead and serum testosterone, luteinizing hormone (LH), and follicle stimulating hormone (FSH). Semen samples were analyzed to determine sperm concentrations, total sperm counts, and the proportion of motile and rapidly motile sperm and sperm with normal morphology. The mean historical blood lead concentration in the subjects was 28.7 micrograms per deciliter (microg/dl). Over 19% of the subjects wanted children in the future, nearly 17% reported no pregnancy after trying for at least 1yr, and over 10% had sought fertility counseling. Current blood lead concentrations varied from 5 to 58microg/dl. Relationships between the blood lead concentrations, and semen quality and serum hormone parameters were assessed by stratifying the subjects into those with blood lead concentrations of below 15, 15 to 24, 25 to 39, and 40microg/dl or greater. The geometric mean sperm concentrations and total sperm counts decreased significantly with increasing blood lead concentrations, even after adjusting for age, smoking, and other potential confounders. A history of difficulty in fathering a child and having sought advice for fertility problems was more common in subjects with current blood lead concentrations below 25microg/dl. When compared with those with blood lead concentrations below 15microg/dl, workers with blood lead concentrations above 40microg/dl had significantly increased risks of having sperm concentrations and total sperm counts below those considered normal by the World Health Organization, odds ratios 8.2 and 2.6, respectively. Sperm morphology, the measures of sperm motility, and serum testosterone, FSH, and LH concentrations were not associated with current blood lead concentrations. Sperm concentration, total sperm counts, and total sperm motility were inversely related to past lead exposure. The authors conclude that blood lead concentrations below the current OSHA standard, 50microg/dl, appear to adversely affect spermatogenesis.