Maternal smoking while pregnant is a plausible risk factor for childhood cancers because many seem to initiate in utero and tobacco-specific carcinogens cross the placenta. Social desirability bias may affect maternal report of smoking in case-control studies and could explain inconsistently observed associations with offspring cancer. Detection of tobacco smoke biomarkers in dried blood spots (DBS), which are increasingly stored by newborn screening programs, may improve retrospective assessment of fetal tobacco exposure. As proof-of-principle, we examined cotinine in DBS of 20 infants enrolled in a pilot study of pregnancy among low-income women. We recruited 107 pregnant women (<30 weeks of gestation) from six Women, Infants, and Children clinics in Minneapolis and St. Paul in 1999. Blood samples obtained at enrollment were tested for total cotinine using gas chromatography/mass spectrometry. Women were then interviewed at 7 months of gestation to determine current smoking habits. DBS were obtained from the Minnesota Department of Health. We tested DBS from 10 infants whose mothers had detectable serum cotinine at baseline and 10 control infants whose mothers had none. One quarter of each DBS was assayed for cotinine using gas chromatography/mass spectrometry; levels were estimated assuming 50 μL blood per sample. Mean cotinine was 29 ng/mL (SD, 7.5), 45 ng/mL (SD, 9.7), and 9 ng/mL (SD, 7.4), respectively, among infants of all smokers, infants of four women who acknowledged smoking at 7 months of gestation, and infants of nonsmokers. These results suggest that DBS analysis may identify infants of women who smoke throughout pregnancy.