Height and weight were measured in a health screening of the population in Nord-Trøndelag, Norway. Correlations for the body mass index were computed for 23,936 pairs of spouses, 43,586 pairs of parents and offspring, 19,151 pairs of siblings, 1,251 pairs of grandparents and grandchildren, 1,146 cognate avuncular pairs, 801 noncognate avuncular pairs, 168 pairs of same-sexed twins, and smaller groups of other types of relatives. Correlations were largely independent of age and age difference within pairs of relatives, suggesting a stable effect of the same set of genes and familial environment throughout adulthood. No effect of convergence during marriage could be detected. Correlations were approximately .20 for parents and offspring, .26 for same-sexed siblings, .20 for opposite-sexed siblings, .58 for monozygotic twins, and close to zero for most second-order relatives. Results from structural equation model-fitting indicate a broad heritability of .4, much of which is due to genetic dominance or other genetic nonadditivity.