OBJECTIVE To assess the possible impact of smoking on body composition at the 34-39 weeks of pregnancy, and its consequences on birth weight. METHODS 31 smokers and 31 non-smokers matched by age, gestational age and percentage of standard weight (PSW). Smoking mothers had 5 cigarettes or more per day during the whole period of pregnancy and non-smoking mothers never smoked. METHODS A cross-sectional study was conducted in an urban Public Health Center. Anthropometry and the deuterium dilution technique were used to determine body composition. The statistical analysis used a t-test for matched pairs to assess significance of the means difference for each variable. RESULTS Suprailiac and subscapular skinfolds were found to be larger in non-smokers; this difference was significant for the suprailiac skinfold (P < 0.05). No significant difference was found in total body fat, total body water and fat-free mass obtained with the deuterium dilution technique, or FFM determined by anthropometry. Infants' birth weight was similar in the two groups of mothers. CONCLUSIONS These findings suggest that pregnancy does not modify the pattern of body fat distribution already present in smokers before conception. Infants' birth weight was similar in both groups, probably due to no difference in fat-free mass and body fat. The physiological effect of nicotine (e.g. on prolactin, micronutrients), seems to be clearer during lactation and infant growth.