Assortative mating by body height and weight is well established in various populations, but its causal mechanisms remain poorly understood. We analyzed the effect of phenotypic assortment and social homogamy on spousal correlations for body height and body mass index (BMI, kg/m²). Our data derived from a questionnaire administered to the adult Finnish Twin Cohort in 1990 (response rate 77%) yielding results from 922 monozygotic and 1,697 dizygotic adult twin pairs who reported information about their body height and weight and that of their spouses. Assortative mating was evident for body height and BMI. For body height, the effects of social homogamy (0.24 in men and 0.29 in women) and phenotypic assortment (0.27 and 0.28, respectively) were about the same. For BMI, the effect of social homogamy was stronger (0.31 in men and 0.28 in women) than the effect of phenotypic assortment (0.13 in both men and women). When assortative mating was taken into account, shared environmental factors had no effect on phenotypic variation in body height or BMI. Our results show that assortative mating needs to be considered in population genetic studies of body height and weight. Am. J. Hum. Biol. 15:620–627, 2003. © 2003 Wiley‐Liss, Inc.