Low levels of high-density lipoprotein cholesterol (HDL-C) are an independent risk factor for atherosclerosis. We investigated the effects of the TaqIB polymorphism of cholesterol ester transfer protein (CETP) on CETP activity and plasma HDL-C levels in random nondiabetic and self-reported diabetic subjects in a population with very low HDL-C levels. The rare B2B2 genotype was associated with significantly higher HDL-C levels and lower CETP activity in random subjects and with higher HDL-C in diabetic subjects. After stratification of random subjects by smoking status, the common B1B1 genotype was associated with lower HDL-C levels than the B2B2 genotype. Although smoking was associated with lower HDL-C, especially in men, HDL-C levels between smokers and nonsmokers were not different in subjects with the B1B2 or B2B2 genotypes. However, smoking (20+ cigarettes/day) was associated with a marked reduction in HDL-C in the B1B1 subjects. The B1B1/smoking interaction was not reflected in a difference in CETP activity. High triglycerides and elevated body mass index (BMI) lower HDL-C. The B2B2 genotype was associated with the highest HDL-C levels, and these levels were significantly lower in the hypertriglyceridemic subjects (>or=50th percentile). The lowest HDL-C levels were seen in hypertriglyceridemic subjects with the B1B1 genotype. Although BMI (>or=50th vs<50th percentile) did not affect HDL-C in B2B2 subjects, a high BMI was associated with markedly lower HDL-C in B1B1 subjects. Thus, HDL-C levels in Turks may be modulated by an interaction between the CETP TaqIB polymorphism and smoking, as well as an interaction with hypertriglyceridemia and BMI.