BACKGROUND/OBJECTIVES: High protein intake has been associated with increased growth. This may be linked to increased concentrations of insulin-like growth factor I (IGF-I), which seems to be influenced by the diet, especially its protein component. The short-term effects of high protein intake in late infancy are not known. The objective was to investigate the effects of high protein intake in the form of whole milk (WM) on growth and IGF-I from 9 to 12 months of age. SUBJECTS/METHODS: Healthy infants (n=83) were randomized to receive either WM or infant formula and fish oil or no fish oil (2 x 2 design). Anthropometric variables, IGF-I concentrations, serum urea nitrogen (SUN) and diet were recorded before and after the intervention. RESULTS: Intake of WM significantly increased the protein energy percentage (PE%; P<or =0.001) and SUN (P=0.01), whereas there was no effect on size. The milk intervention increased IGF-I in boys (P=0.034) but not in girls. Intake of fish oil had no effect on the outcomes. Including all infants in the analysis there was a significant correlation between weight and IGF-I at 12 months (r=0.316, P=0.017), and PE% was positively associated with IGF-I after adjusting for sex and breastfeeding at both 9 (r=0.329, P=0.015) and 12 months (r=0.272, P=0.044). CONCLUSIONS: Randomization to WM had no overall effect on growth. However, the positive effect of WM on IGF-I in boys and the positive association between PE% intake and IGF-I at 9 and 12 months is consistent with the hypothesis that a high milk intake stimulates growth.