The contributions of shared genes and shared environments to familial aggregation of coronary heart disease risk factors were investigated by genetic and epidemiologic analysis of 434 adult female twin pairs from the Kaiser-Permanente Twin Registry in Oakland, California, during 1978 and 1979. Initial estimates of genetic heritability were statistically significant for serum levels of high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, triglycerides, and Quetelet index, but were only marginally significant for systolic and diastolic blood pressures. These estimates were biased, however, because sisters in the same identical twin pair were more similar than sisters in the same fraternal twin pair not only with respect to shared genes but also with respect to shared environmental and behavioral influences. Heritability was estimated again after adjusting for shared environmental and behavioral effects by multiple regression analysis. Genetic heritability remained significant for HDL cholesterol (0.66), LDL cholesterol (0.88), triglycerides (0.53), and relative weight (0.55) but not for systolic (0.42) and diastolic (0.25) blood pressures. The strong genetic components of the levels of LDL cholesterol, HDL cholesterol, and relative weight may in part explain why some women have high levels of these coronary disease risk factors despite following recommended health behaviors.