The relationships among cigarette smoking, hemoglobin concentration, and carboxyhemoglobin concentration were examined using data from the Second National Health and Nutrition Examination Survey. Among women, smokers had a mean (+/- SE) hemoglobin level of 137 +/- 0.4 g/L, significantly higher than the mean hemoglobin level of 133 +/- 0.5 g/L for never-smokers. Among men, the mean hemoglobin levels for smokers and never-smokers were 156 +/- 0.4 and 152 +/- 0.5 g/L, respectively. No significant difference in mean hemoglobin was noted between ex-smokers and never-smokers. Mean hemoglobin levels and carboxyhemoglobin levels increased progressively with the number of cigarettes consumed per day. Cigarette smoking seems to cause a generalized upward shift of the hemoglobin distribution curve, which reduces the utility of hemoglobin level to detect anemia. Among women of comparable socioeconomic status, the prevalence of anemia was 4.8% +/- 0.6% among smokers, compared with 8.5% +/- 1.2% among never-smokers. This study suggests that minimum hemoglobin cutoff values should be adjusted for smokers to compensate for the masking effect of smoking on the detection of anemia.