Longevity and morbidity and death from myocardial infarction were examined in eight kindreds with familial hypobeta lipoproteinemia and in 18 kindreds with familial hyperalpha lipoproteinemia. Expectation of life for males and females from kindreds with hypobeta lipoproteinemia was 9 and 12 years longer (p less than or equal to 0.002) than that indicated by population statistics for U.S. white populations, whereas expectation of life for males and females from kindreds with hyperalpha lipoproteinemia was 5 and 7 years longer (p less than 0.02). Morbidity from myocardial infarction in 115 living first-degree adult relatives of probands with hypobeta and hyperalpha lipoproteinemia and in 364 living first-degree adult relatives of normolipemic spouse controls were compared. Nonfatal myocardial infarction (MI) was reported for 18 of 364 (5 per cent) relatives of normal spouse controls and in 0 of 115 relatives of hypobeta and hyperalpha subjects (p less than 0.05). The ratios (mean+/-S.E.) of C-LDL to C-HDL in familial hypobeta and hyperalpha lipoproteinemia were 0.79+/-0.06 and 1.21+/-0.06, as compared to 2.41+/-0.12 in a control population (p less than 0.001). If high-density lipoproteins confer protection against development of atherosclerosis, whereas low-density lipoproteins have opposite effects, then we speculate that the low ratio of C-LDL:C-HDL may be related to prolonged longevity and decreased morbidity from myocardial infarction in familial hypobeta and hyperalpha lipoproteinemia.