We measured nicotine metabolites (cotinine and trans-3'-hydroxycotinine) in meconium of infants of passive or active smokers as a direct marker of fetal exposure to tobacco smoke. Meconium was collected from 55 infants whose mothers were nonsmokers, passive smokers, or light or heavy active smokers. Nicotine metabolite concentration (NMC) in meconium was analyzed by radioimmunoassay and gas chromatography-mass spectrometry. Radioimmunoassay showed the following mean meconium NMCs (in nanograms per milliliter); nonsmoker, 10.9; passive smoker, 31.6; light active smoker; 34.7, and heavy active smoker, 54.6. Analysis of available samples by gas chromatography-mass spectrometry confirmed the presence of cotinine. Correlation between meconium NMC and the degree of maternal smoking was 0.54 (p < 0.001). Meconium NMCs in infants of passive and active smokers were significantly higher than in those of nonsmokers (p < 0.05). Meconium NMC in passive smokers was not significantly different from that in light active smokers (p > 0.05). Thus exposure of the fetus to tobacco smoke is substantial, even by passive maternal smoking. Meconium analysis for nicotine metabolites may be useful for clinical and research purposes.