OBJECTIVES. Higher levels of serum cotinine (a metabolite of nicotine) have been found in Black smokers than in White smokers even after self-reported cigarette dose was controlled. It is unknown whether higher cotinine levels in Black pregnant smokers may increase the risk of delivering an infant of reduced birthweight. METHODS. We analyzed serum cotinine levels of 374 Black and 829 White women who smoked during pregnancy and who delivered between April 1964 and April 1967. Racial differences in the relationship between cotinine and birthweight were examined. RESULTS. Cotinine levels were 27.4 ng/mL higher in Black smokers after cigarette dose and confounding variables were controlled. Blacks had higher cotinine levels than Whites at each dose. No significant racial differences in the rate of decrease in birthweight per nanogram of cotinine per milliliter were found. CONCLUSIONS. Our results confirm previous research showing higher cotinine levels at each smoking dose in Black smokers than in White smokers. Because there was no difference in the rate of decrease in birthweight due to cotinine, our results suggest that cigarette smoking among Blacks may have a greater effect on birthweight than it does among Whites.