Pregnant women who are underweight for height and gestational age are at risk of delivering infants weighing less than infants born to women of standard weight. This study demonstrated that a nutrition intervention program geared toward these high-risk women improved infant outcome. A group of 57 underweight and failure-to-gain pregnant women (treated) who received extensive nutrition counseling and follow-up from a nutritionist throughout their pregnancy was compared with a similar group of 52 underweight pregnant women (control) who received no counseling. Women in the treated group gained significantly more weight during their pregnancy than did controls. The treated women also delivered infants who averaged 300 g heavier than those born to the control group, a statistically significant difference. Other factors reported to compromise infant outcome such as ethnic background, income status, age, and smoking did not significantly affect infant birth weight.