CONTEXT: Low levels of high-density lipoprotein (HDL) cholesterol are inversely related to cardiovascular risk. Whether this is a causal effect is unclear. OBJECTIVE: To determine whether genetically reduced HDL cholesterol due to heterozygosity for 4 loss-of-function mutations in ABCA1 cause increased risk of ischemic heart disease (IHD). DESIGN, SETTING, AND PARTICIPANTS: Three studies of white individuals from Copenhagen, Denmark, were used: the Copenhagen City Heart Study (CCHS), a 31-year prospective general population study (n = 9022; 28 heterozygotes); the Copenhagen General Population Study (CGPS), a cross-sectional general population study (n = 31,241; 76 heterozygotes); and the Copenhagen Ischemic Heart Disease Study (CIHDS), a case-control study (n = 16,623; 44 heterozygotes). End points in all 3 studies were recorded during the period of January 1, 1976, through July 9, 2007. MAIN OUTCOME MEASURES: Levels of HDL cholesterol in the general population, cellular cholesterol efflux, and the association between IHD and HDL cholesterol and genotype. RESULTS: Heterozygotes vs noncarriers for 4 ABCA1 mutations (P1065S, G1216V, N1800H, R2144X) had HDL cholesterol levels of 41 mg/dL (interquartile range, 31-50 mg/dL) vs 58 mg/dL (interquartile range, 46-73 mg/dL), corresponding to a reduction in HDL cholesterol of 17 mg/dL (P