To evaluate the interaction between the angiotensin-converting enzyme (ACE) gene insertion/deletion (I/D) polymorphism and gender with individual blood pressure response to hydrochlorothiazide (HCTZ) in hypertensives, we enrolled 829 mild-moderate hypertensive patients. All subjects were given HCTZ (12.5 mg) orally each day for 6 weeks. A total of 776 patients completed the study. There was statistically significant interaction between the effects of genotype and gender on systolic (P = 0.002) and diastolic (P = 0.048) response after adjusting for covariables. Moreover, in each gender, the genotype that was associated with the greatest blood pressure response to HCTZ (DD homozygotes in men and II homozygotes in women) was also associated with the greatest increase in serum ACE activity in response to HCTZ. The results suggest that the I/D polymorphism of the ACE gene is associated with interindividual differences in the blood pressure response to a low dose of a diuretic in a gender-specific manner in the Han Chinese population with hypertension.