Plasma low density lipoproteins (LDL) comprise multiple discrete subclasses, differing in size, density, and chemical composition. Gradient gel electrophoresis of LDL has demonstrated three common subclass patterns based on the predominant LDL subclass: large LDL, designated subclass pattern A; small LDL particles, designated subclass pattern B; and an intermediate pattern. Genetic studies have demonstrated that these patterns are inherited, but several lines of evidence suggest that environmental factors are important in the phenotypic expression of the pattern. The LDL B pattern is associated with increased levels of plasma triglyceride, reduced high density lipoprotein (HDL), and obesity. To better define the role of environmental factors on LDL phenotypic expression, we determined LDL patterns in patients with the acquired immunodeficiency syndrome (AIDS), an infection characterized by hypertriglyceridemia and weight loss. Similar to previous studies, plasma triglyceride levels were increased, whereas plasma cholesterol, LDL cholesterol, and HDL cholesterol levels were decreased in the AIDS subjects compared to those in age-matched controls. The percentage of AIDS subjects with the LDL B phenotype was increased 2.5-fold, demonstrating an increased prevalence of the LDL B phenotype in an acquired form of hypertriglyceridemia. For each LDL phenotype in AIDS, serum triglyceride levels were higher than the same phenotypic pattern in controls, with the most marked elevations in triglycerides found in AIDS subjects with the LDL B phenotype. In contrast to what was observed in controls, HDL cholesterol levels were decreased in all AIDS subjects and were unrelated to LDL pattern. Total and LDL cholesterol levels were higher in controls with the LDL B phenotype than in those with the LDL A phenotype, but there was no difference in total and LDL cholesterol in AIDS subjects with LDL B compared to A. On multiple regression analysis in subjects with AIDS, plasma triglyceride levels, age, and HDL cholesterol all contribute to the occurrence of the LDL B phenotype, but elevations in plasma triglyceride levels are the strongest independent predictor. Body mass index was not a predictor of LDL B phenotype in AIDS. These results suggest that disturbances in triglyceride metabolism that are caused by AIDS lead to the appearance of the LDL subclass B phenotype and provide further evidence that environmental or disease states that perturb lipid metabolism can produce an increased prevalence of the LDL B phenotype.