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Body density through underwater weighing and 6 skinfolds (triceps, biceps, subscapular, suprailiac, abdominal and medial calf) were measured in 871 biological and adopted siblings of French descent from the greater Quebec city area. These individuals belonged to 49 sibships of adopted sibs, 71 sibships of unrelated individuals living together including adoptees, 35 «sibships» of first degree cousins, 229 sibships of biological brothers and sisters, 66 sibships of DZ twins and 83 sibships of MZ twins. Age of subjects ranged from 8 to 26 years. Age and sex accounted from 14% to 48% of the variance in body composition measurements, with the exception of fat free weight (71%). Residual scores of age and sex were submitted to analysis of variance and correlation analysis. The ratio of the between sibships to the within sibship means of squares reached significance (p ≤ .05) for almost all measurements in the MZ, DZ, biological and cousin sibships, for the skinfolds but not for body density, fat free weight and the ratio of subcutaneous fat to total fat mass in the unrelated sibships, and for none of the measurements in the adoptive sibships. Interclass correlations could be computed in a maximum of 80 pairs of adopted sibs, 120 pairs of unrelated sibs, 95 pairs of cousins, 370 pairs of biological sibs, 69 pairs of DZ twins and 87 pairs of MZ twins. None of the coefficients reached 0.20 (p < 0.05) in the pairs of adopted or unrelated sibs. On the other hand, correlations were often significant in the pairs of cousins, particularly for the skinfold measurements. The same coefficients were generally significant and increasing from the pairs of brothers and sisters to the pairs of DZ twins and MZ twins. This pattern was particularly clear for several individual skinfolds, the extremity and the trunk skinfolds, the ratio of extremity to trunk subcutaneous fat and fat free weight. Genetic effects reaching about 0.40 and above were found for measurements of subcutaneous fat, fat distribution, body density and fat free weight. Twin data yielded higher and often unrealistic estimates of heritability probably because of the differential effects of the common environment in MZ and DZ pairs.