India is a major contributor to the global public health burden of diabetes. We have undertaken a family study of large multiplex families from Chennai, South India, and report on the familial aggregation of quantitative traits associated with type 2 diabetes mellitus in these pedigrees. Five hundred twenty-four individuals older than 19 years from 26 large multiplex pedigrees were ascertained. Detailed questionnaires and phenotype data were obtained on all participating individuals including fasting blood glucose, fasting insulin, lipid profiles, height, weight, and other anthropometric and clinical measures. Heritability estimates were calculated for all quantitative traits at the univariate level, and bivariate analyses were done to determine the correlation in genetic and environmental control across these quantitative traits. Heritability estimates ranged from 0.21 to 0.72. The heritability estimates for traits most directly related to type 2 diabetes mellitus were 0.24 +/- 0.08 for fasting blood glucose and 0.41 +/- 0.09 for fasting insulin. In addition, there was evidence for common genetic control for many pairs of these traits. These bivariate analyses suggested common genes for fasting insulin and central obesity measures (body mass index, waist, and hip), with complete genetic correlation between fasting insulin and waist. Quantitative traits associated with type 2 diabetes mellitus have heritabilities suggestive of some familial or genetic effect. The evidence for pleiotropic control of insulin and central obesity-related traits supports the presence of an insulin resistance syndrome in South Asians with a tendency for central obesity.