The effects of smoking during pregnancy on maternal body composition and the fetoplacental unit were investigated serially in well-matched groups of 29 normal, healthy primigravid smokers and 31 nonsmokers. The babies of smokers were lighter by an average of 138 g, and there was a greater proportion of small-for-dates infants among smokers compared with nonsmokers. There was a failure in expansion of mean plasma volume and total body water in patients who smoked throughout pregnancy compared with nonsmokers. No differences were demonstrated in serum protein, intravascular protein mass, serum albumin, intravascular albumin mass, serum electrolytes, serum osmolality, urinary estriol excretion, fetal biparietal diameter, or weekly growth rate between the 2 groups. The concentration of serum heat-stable alkaline phosphatase at 34 and 38 weeks' gestation, during the second stage of labor, and in the early puerperium was significantly higher in smokers. It is postulated that smoking unfavorably alters maternal body composition which is manifested in poor overall performance with the consequent production of smaller infants. The effects of tobacco smoke on the fetus may possibly be mediated via the placenta.