Insertion (I)/deletion (D) polymorphism of the ACE gene has been reported to be involved in various cardiovascular diseases. We investigated prospectively whether the response to the ACE inhibitor fosinopril varied according to the ACE genotype in previously untreated Greek hypertensive patients. After a 4-week observation period, fosinopril was administered at a dose of 20 mg daily and blood pressure was measured weekly for 6 months. The study population consisted of 104 hypertensive patients (46 male, 58 female). There were no differences in age, gender, body mass index, and pretreatment blood pressure levels among patients with the DD, ID, and II genotypes (n= 42, 30, 32, respectively). The reduction in systolic blood pressure was significantly greater in patients carrying the DD compared to II or ID genotypes (5.6 +/- 3.1 vs. 3.1 +/- 1.1 or 3.6 +/- 2.2, respectively; ANOVA, p < 0.05). The reduction in diastolic blood pressure was also significantly greater in DD hypertensives compared with II or ID (8.9 +/- 6 vs. 5.5 +/- 3.4 or 5.8 +/- 4, respectively; ANOVA, p < 0.05). The age and BMI were not correlated with the changes in SBP or DBP that were observed after fosinopril administration. In conclusion, the ACE gene genotype was shown to influence the response to fosinopril in hypertensive patients.