Data relating to weight of infants from 23 developing countries and two western industrialized populations taken from the literature were compared. Growth in the developing countries was extremely variable, but was poor compared to western standards such that by 12 months of age mean weight in all 23 populations was below the NCHS 50th percentile (P < 0.001) and in 11 populations mean weight was below the 10th percentile. Birth weight was not related to growth increment in the first 3 months. Multiple regression analysis showed birth weight and growth increments in the first four 3-month periods post-partum to be highly significantly related to weight at 12 months (P< 0.0001 in all cases). Growth increment in the first 3 months had the greatest effect out of these variables; the effect of the other four variables was similar. These patterns demonstrate the variable impact of environmental factors on growth in the first year of life.