The relationship of antenatal weight gain to pregnancy outcome was studied in 362 pairs of underweight women (less than 90% Metropolitan Relative Weight) and normal weight (90% to 110%) women who were matched for age, occupation, height, parity, race, and smoking habits. The mothers, selected from a study of pregnancy outcome in 1,080 middle-class women, had early and regular prenatal care. Regression analyses within each initial weight category revealed that there was no relationship between initial weight of mothers and birth weight of their infants, but birth weight increased significantly with antenatal weight gain (p less than .0001). A 1-kg maternal increase in weight was associated with a 12.85-gm increase in birth weight in offspring of underweight women and an 8.59-gm increase in offspring of normal weight women. Underweight women had significantly larger prenatal weight gains (9.3 vs. 8.5 kg), but there were no statistically significant differences in mean birth weight, gestational age, or Apgar scores between infants of women in the two weight groups. Very underweight women (less than 80% MRW) had lower-birth-weight infants, more preterm infants, and more infants with medical complications. Antenatal weight gain accounted for the greatest variation in birth weight among infants of underweight and normal weight women. When underweight mothers gained less than 9 kg, their infants had mean birth weights 361 gm less than the mean birth weights of infants of underweight women who gained more weight.