To assess the relationship between the angiotensin converting enzyme (ACE) gene I/D polymorphism, blood pressure (BP) and family history of hypertension, 133 hypertensive subjects (mean age 50 +/- 9 years, 78 males, 55 females) were selected according to both casual supine BP > 140/90 mmHg and ambulatory BP > 134/88 mmHg. Drug treatment was discontinued 2 weeks before entering the study. Subjects with myocardial ischemia, as well as those with "white coat" hypertension, were excluded. The study population was subclassified according to age < or = 50 years. Polymerase chain reaction was used to detect the I/D polymorphism of the ACE gene, and the DD genotype was analysed twice. The frequencies of the I and D allele were 42 and 58%, and the distribution of the ID+ II and DD genotypes were 69 and 31% respectively. No significant relation was found among ACE genotypes (DD vs ID+ II) and casual systolic or diastolic BP as well as ambulatory BP, both in the whole study population and in the subpopulation < 50 years old. No difference was found also in the distribution of dippers and no dippers, as well as in the distribution of subjects with a positive family history in the whole sample and hypertensives < 50 years old.