Many nutritional intervention programmes throughout the world have targeted food supplements towards women during the "at risk" periods of pregnancy and lactation. Some of these programmes, together with a number of small-scale scientific studies, have attempted to evaluate the efficacy of supplementation in terms of birth outcome or lactational performance. After reviewing the available evidence it is concluded: (a) that supplementation during late pregnancy can have a significant beneficial effect on birthweight in women who are genuinely "at risk" due to an inadequate home diet; (b) that statistical projections would predict that the increase in birthweight should be accompanied by a significant decrease in neonatal mortality; (c) that supplementation during lactation is most unlikely to increase breast-milk output or significantly improve its composition except perhaps in extremely malnourished women. Since pregnant women form a small and easily identifiable target group, and since the potential benefits of extra food may be substantial, it is recommended that future efforts are focussed in this direction. Large-scale effectiveness and cost-benefit trials are required with neonatal mortality as the primary outcome.