This study investigated the association between maternal cigarette smoking and fetal growth, evaluated by longitudinal ultrasound examinations and by neonatal anthropometric measurements. The investigation was carried out in a healthy population of affluent Scandinavian women, parity 1 and 2, who were selected consecutively and prospectively, and with term, normal pregnancies. Three hundred and six non-smoking, 242 light-smoking and 308 heavy-smoking mothers and their newborns were examined. Ultrasound measurements were performed in pregnancy weeks 17, 25, 33 and 37. Biparietal diameter (BPD), mean abdominal diameter (MAD) and femur length were recorded. The negative effect on fetal growth from maternal smoking was found to affect the male fetus proportionally more than the female. Boys born to heavy-smoking mothers had a weight reduction of 8.2% and a lower fat accretion (as measured by subscapular skinfold) of 12%, whereas girls had a weight and fat reduction of 4.8% and 2% respectively. In boys (but not girls) born to smokers, head circumference was significantly smaller, also reflected by significantly smaller mean BPD measurements recorded from pregnancy week 18 onwards. The MAD measurements became successively more negatively affected in the second half of pregnancy in both males and females. A greater intrauterine growth velocity and a different hormonal milieu are suggested as possible explanations of the greater male susceptibility.