From 10 to 34 days of age, the weight gain was studied in 134 low birth weight (LBW) infants (birth weight range 0.99-1.90 kg: gestational age range 28-38 weeks) on three different diets: (1) human milk; (2) a “humanized” milk formula; (3) a high protein, high CL low fat cow's milk foumula. The caloric intake (keal/kg/24 hr) was always about 120 between 10 and 20 days, and about 140 thereafter. Six experimental groups were separated according to the three diets, and whether infants were “large for date” (LFD, i. e., with birth weight > the 25th percentile for gestational age) or “small for date” (SFD, i. e., with birth weight ≤ the 25th percentile, age 50'; of subjects ≤ the 10th percentile). Weight gain curves were calculated by regression analysis using the least square method according to a polynomial model. In each group, an exponential relationship of weight gain with time was found. In SFD infants weight gain was slightly but significantly (P < 0.001) father than in LFD babies on diet 3, not on diets 1 and 2. Either in LFD or in SFD infants weight gain was markedly faster on diet 3 as compared to diet 1 or 2 (P < 0.001), and slightly but significantly (P < 0.001) faster on diet 1 as compared to diet 2. Previous findings of a faster weight gain of LBW newborns on high protein formulas, as compared to human or “humanized” milk foumulas, were therfore confirmed. The following main conclusions and speculations were also made: in studies on growth refeeding. LFD and SFD newborns should be discriminated, and statistiscal methods able to describe a possible nonlinearity of growth curves should be used; SFD babies presumably need more proteins than LFD infants for optimal weight gain in the early weeks of life.