These clinical trial results are the first, to our knowledge, from a prospective, randomized, and controlled experiment demonstrating that a reduction of smoking during pregnancy improves the birth weight of the infant. Nine hundred thirty-five pregnant smokers were randomly assigned to treatment and control groups; the former received smoking intervention. At the eighth month of pregnancy, differences between the two groups in salivary thiocyanate level and reported smoking were statistically significant. For single, live births, the treatment group infants had a mean birth weight that was 92 g heavier and were 0.6 cm greater in length than the control group infants. The decrement in weight related to smoking cannot be fully explained by gestational age. The findings suggest that some fetal growth retardation can be overcome by the provision of antismoking assistance to pregnant women.