Reproductive hormones are important determinants of sperm production and male sexual function. Early animal and human research during the 1970s suggested cigarette smoking could adversely affect male hormone profiles although subsequent studies did not reach consensus. BMI, age, and alcohol intake were recognized as important covariates of this relationship in the 1980s but studies still did not reach consensus. Exposure misclassification may explain part of the variability in findings, including misclassification due to environmental tobacco smoke (ETS). The objective of this work was to use a biomarker of cigarette exposure (nicotine metabolite in blood serum) to more accurately describe effects of cigarette smoke on male reproductive hormones in a cohort of healthy working men. This work was based on 192 men enrolled as a subset from a larger occupational health study. Each participant gave informed consent and was interviewed with a questionnaire tool about general health, work, ETS exposure, and lifestyle habits. Each was measured for height and weight and provided a single blood specimen. Blood serum was evaluated for reproductive hormones. DPC IMMULITE 1000 platform was used for the assays. Free Androgen Index (FAI) was derived (FAI= 3.467 x total testosterone / SHBG). Selected categorical characteristics (Table 1) and continuous characteristics (Table 2) of the study subjects are shown below. Over 92% of all men reported environmental cigarette smoke exposure at work or home. After controlling for BMI, age, alcohol, and specific work exposure in multiple linear regression modeling, only androstenedione showed a statistically significant increase with increasing nicotine metabolite in the blood (p<0.0001). Using a biomarker of exposure that allows modeling total cigarette exposure (smoking and indirect ETS), and controlling for important covariates, it was found that androstenedione levels increased in a dose-dependent fashion with cigarette smoke exposure in this cohort of healthy men. This confirms previous studies finding androstenedione changes associated with smoking but adds important new information related to total exposure including ETS after adjusting for important confounders. Further modeling will determine the amount of change attributed to each source of exposure.