We investigated the associations between a polymorphism in the melanocortin 4 receptor (MC4R) gene and changes in body size and composition from childhood to adulthood in the Québec Family Study. Ninety-one subjects (43 males) less than 18 years of age (mean age 13.5 +/- 2.4 years; range 8.4-17.8 years) at baseline were re-measured 11.2 years later on average. The anthropometric variables analyzed were height, weight, body mass index, percent body fat, sum of skinfolds, fat mass index, and fat free mass index (FFMI). All variables were adjusted for age and sex. The subjects were genotyped for the MC4R C-2745T polymorphism. Forty-five subjects were homozygotes for the common allele (C/C), 36 were heterozygotes (C/T) and 10 were homozygotes for the rare allele (T/T). The rare allele was associated with increased height at baseline as well as at the follow-up visit. Although FFMI tended to increase more in subjects carrying the rare allele, no significant differences were found for the changes over time for the other phenotypes. These results suggest that DNA sequence variation in the MC4R locus may contribute to the gain in body height from childhood to adulthood.