Diet and growth were studied prospectively in 78 boys ages 6 months to 14 years. All boys were well nourished and not grossly obese. Pubertal development was evaluated at age 14 years, and the results correlated with diet and early growth. No nutrients were significantly correlated with the stage of pubertal development. Boys with more advanced pubic hair development (Tanner greater than 3) and longer penile length (greater than or equal to 10 cm) had been significantly heavier at ages 6 months, 2 years, and 4 years (all P less than 0.025). Muscle mass, as estimated by the cross-sectional muscle area of the upper arm, had been significantly greater in the early maturers at the same ages (P less than 0.025 in seven of nine comparisons). Although the more sexually mature boys also had been taller and had had larger skinfolds at virtually all measurements from age 6 months to 4 years, the differences were less pronounced. In this adequately nourished male population, body size in the first years of life was significantly correlated with the timing of puberty.