The purpose of this study was to analyze the association between 24-h blood pressure parameters, insertion/deletion polymorphism of the angiotensin I converting enzyme gene and the ABO blood group system in a sample of the general Czech population. Fourteen parameters describing the 24-h blood pressure readings were obtained by analyzing blood pressure records in 243 volunteers, 119 men and 124 women. These parameters were adjusted for sex and body mass index (BMI) by means of multiple regression test. All subjects were genotyped for the insertion/deletion polymorphism of angiotensin I converting enzyme (I/D ACE) gene and routinely examined for the blood group phenotype in the ABO system. An association was found between the interaction of I/D ACE gene polymorphism and ABO blood group system on the one hand and mean values of systolic (p=0.016) or mean arterial (p=0.027) blood pressures and the phase shift of 24-h BP rhythm (p=0.036) on the other hand. Among three I/D ACE variants the DD genotype was associated with the highest values of mean blood pressure in blood group A and AB carriers. The same genotype was associated with the lowest blood pressures in blood group B and O carriers. In subjects with the DD genotype, the earlier daily position of the maximum of 24-h BP rhythm was found in blood group B, AB and O carriers. On the contrary, blood group A was associated with the latest position of maximum of the 24-h BP rhythm in the DD genotypes.