This study estimated the genetic and environmental determinants of plasma leptin and insulin levels and of obesity-related phenotypes. Included in this analysis were family members from 80 families living in kibbutz settlements, who participated in two examinations 8-10 years apart. We estimated that polygenes explained 30-50% of the adjusted leptin and insulin levels and 30-70% of the anthropometric phenotypes. This study demonstrated a significant genetic influence on longitudinal changes in leptin and BMI (h(2) = 0.45) and small-to-moderate heritability estimates for changes in insulin and other obesity-related phenotypes. In bivariate genetic analyses, we observed positive genetic correlations between leptin and anthropometric phenotypes, suggesting that shared effects of the same sets of loci account for 20-30% of the additive genetic variance in these pairs of variables. Shared genetic factors also account for 20-25% of the additive genetic variance in insulin-anthropometric pairs of variables.