Patients with non-insulin-dependent diabetes mellitus (NIDDM) are known to have abnormalities in their low density lipoprotein (LDL) subclass pattern with a preponderance of small dense LDL. The present study was performed to define the roles of lipolytic enzymes (hepatic and lipoprotein lipase) and cholesteryl ester transfer protein (CETP) in determining the distribution of LDL subfractions in these patients. LDL subfractions were measured by density gradient ultracentrifugation in 137 patients with NIDDM (75 male, 62 female) and 140 matched controls (80 male, 60 female). The male diabetic patients had a lower concentration of LDL-I (P < 0.01) and a higher concentration of LDL-III than the controls (P < 0.01). In the female diabetic patients, both LDL-I (P < 0.001) and LDL-II concentrations (P < 0.05) were significantly lower than the controls whereas LDL-III was increased (P < 0.001). Hepatic lipase (HL) was significantly increased in both the male and female diabetic patients (P < 0.01, P < 0.05, respectively) compared to their controls. No significant changes were seen in plasma lipoprotein lipase (LPL) and CETP activity. On multivariate analysis, plasma triglyceride (TG), CETP and HL accounted for 10, 5 and 3% of the variability in LDL-III, respectively, in the diabetic patients (adjusted R2 = 0.18, P = 0.0003). Our findings would support the hypothesis that plasma triglyceride influences LDL particles through a cycle of lipid exchange via the action of CETP. LDL become enriched in triglyceride and are then acted on by HL to produce a population of small dense lipid-poor LDL.