Hypertension is a multifactorial disease, in which genetic factors play an important role. This study was carried out to determine angiotensin-converting enzyme levels and angiotensin-converting enzyme gene polymorphism in Turkish hypertensive patients, and to establish whether there is an association of angiotensin-converting enzyme gene polymorphism with clinical and echocardiographic parameters. We have investigated the association among the allelic distribution of the insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme gene identified by polymerase chain reaction, angiotensin- converting enzyme activity determined spectrophotometrically, cardiac morphology and function assessed by means of echocardiography. Distribution of angiotensin-converting enzyme gene I/D polymorphism and allele frequencies in hypertensive patients was not significantly different from controls. D allele frequency was 51.7% in hypertensives vs. 51.9% in controls and I allele 48.3 vs. 48.1%, respectively. The level of angiotensin-converting enzyme activity was significantly higher in the patients homozygotes for D allele (DD = 59.93 U/l) than in heterozygotes (ID = 39.49) and in homozygotes for I allele (II = 40.28 U/l). In addition to these, the level of angiotensin-converting enzyme activity was significantly lower in the ID and especially II patients receiving ACE inhibitors than the others. Also, it was determined that left atrium diameter was larger in the patients homozygotes for I allele than the others.