Studies on nutritional supplementation of pregnant mothers have observed small but sometimes significant effects on fetal growth. Previously we also reported significant changes in physical resemblance of siblings at birth due to this kind of supplementation, using data collected by the late Dr. Bacon Chow and associates. This suggested that components of variation in physical growth may change as a result of supplementation. To explore this further, we sought the effects of nutritional supplementation on somatic growth, holding constant the birth measurements of a previous (unsupplemented) sibling. Only head circumference at birth came close to showing a statistically significant (P less than 0.06) supplement effect on its mean (+ 0.36 cm) when the measurement of the first infant was held constant. Both increases and decreases in weight for length as assessed by Rohrer's Index (wt/l3) occurred with supplementation. Increases in the index occurred by a significant (P less than 0.01) gain in birth weight (238 g) between 1st and 2nd siblings in the supplement group but not in the placebo group. Decreases in this index resulted from a significant gain in birth length (1.3 cm) in supplemented families with a previous (untreated) sibling having a below average length for average weight. These non-linear effects on somatic growth suggest that responses to maternal nutritional supplementation may be more effective in a subgroup of the population where the normal adaptive responses to low energy protein intakes may have failed or where nutritional conditions over pregnancy may have varied considerably.