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The purpose of the study was to determine the effect of diabetes on the regulation of postprandial cholesterol metabolism. Four groups of patients (n = 8 for each group) were examined: Type 2 diabetic patients with and without hypercholesterolemia and nondiabetic subjects with and without hypercholesterolaemia. Serum lipoproteins, lipoprotein composition, cellular cholesterol, and cellular cholesterol synthesis were measured before and 4 h after a high calorie meal. The BMI for the hypercholesterolaemic diabetic patients of 31.5 ± 0.95 (SEM) was significantly higher than that for the control group of 26.2 ± 1.0 (p < 0.01). Fasting triglyceride levels were significantly higher in the normocholesterolaemic and hypercholesterolaemic diabetic patients and in the hypercholesterolaemic non-diabetic subjects (1.45 ± 0.22, 2.27 ± 0.34, and 1.58 ± 0.18 mmol l−1, respectively) compared with normocholesterolaemic non-diabetic subjects (0.75 ± 0.12 mmol l−1: p < 0.01). The normocholesterolaemic and hypercholesterolaemic diabetic subjects had significantly lower fasting serum high density lipoprotein (HDL) (1.06 ± 0.08 and 1.04 ± 0.06 mmol l−1) compared to the corresponding non-diabetic groups (1.29 ± 0.11 and 1.45 ± 0.17 mmol l−1, p < 0.05). The esterified/free cholesterol ratio of very low density lipoprotein (including chylomicrons VLDL-C) decreased postprandially in all groups with an overall decrease of 1.33 to 0.83 (p < 0.01). Fasting cellular cholesterol in mononuclear leucocytes from normocholesterolaemic diabetic patients was similar to that for hypercholesterolaemic diabetic (36.8 ± 1.2 vs 40.6 ± 5.5 mg g−1 protein) and non-diabetic subjects (36.7 ± 6.8 mg g−1 protein) and significantly greater than cholesterol in cells from control subjects (29.7 ± 1.5 mg g−1 protein, p < 0.05). In cells from control subjects only, there was a significant postprandial increase in cholesterol to 39.4 ± 5.2 mg g−1 protein (p < 0.05) and a corresponding postprandial reduction in cholesterol synthesis from 149 ± 34 to 102 ± 33 nmol g−1 cell protein (p < 0.05). These results demonstrate a lack of correlation between serum and cellular cholesterol in diabetic patients and an inability to suppress cellular cholesterol synthesis postprandially in these patients. The differences may, in part, explain the increased deposition of cholesterol in atheromatous plaques in normocholesterolaemic diabetic patients.