OBJECTIVE: To test the efficacy in terms of birth weight and infant survival of a diet supplement programme in pregnant African women through a primary healthcare system. DESIGN: 5 year controlled trial of all pregnant women in 28 villages randomised to daily supplementation with high energy groundnut biscuits (4.3 MJ/day) for about 20 weeks before delivery (intervention) or after delivery (control). SETTING: Rural Gambia. SUBJECTS: Chronically undernourished women (twin bearers excluded), yielding 2047 singleton live births and 35 stillbirths. MAIN OUTCOME MEASURES: Birth weight; prevalence of low birth weight (< 2500 g); head circumference; birth length; gestational age; prevalence of stillbirths; neonatal and postneonatal mortality. RESULTS: Supplementation increased weight gain in pregnancy and significantly increased birth weight, particularly during the nutritionally debilitating hungry season (June to October). Weight gain increased by 201 g (P < 0.001) in the hungry season, by 94 g (P < 0.01) in the harvest season (November to May), and by 136 g (P < 0.001) over the whole year. The odds ratio for low birthweight babies in supplemented women was 0.61 (95% confidence interval 0.47 to 0.79, P < 0.001). Head circumference was significantly increased (P < 0.01), but by only 3.1 mm. Birth length and duration of gestation were not affected. Supplementation significantly reduced perinatal mortality: the odds ratio was 0.47 (0.23 to 0.99, P < 0.05) for stillbirths and 0.54 (0.35 to 0.85, P < 0.01) for all deaths in first week of life. Mortality after 7 days was unaffected. CONCLUSION: Prenatal dietary supplementation reduced retardation in intrauterine growth when effectively targeted at genuinely at-risk mothers. This was associated with a substantial reduction in the prevalence of stillbirths and in early neonatal mortality. The intervention can be successfully delivered through a primary healthcare system.