This study was undertaken to evaluate the association-of polymorphism of angiotensin I converting enzyme (ACE) gene with casual and ambulatory blood pressure in the Chinese population. Genomic DNA was amplified by polymerase chain reaction (PCR) using primers flanking the polymorphic region in intron 16 of the ACE gene. Alleles were detected on agarose gels stained with ethidium bromide. Casual blood pressure was measured in the 133 normotensive and 122 essential hypertensive subjects; of the essential hypertensive subjects, 65 patients underwent non-invasive ambulatory blood pressure. There was no significant difference of ACE genotype and allele frequencies between the hypertensive and normotensive subjects. Non-significant differences were demonstrated in gender, age, casual blood pressure according to the ACE gene genotypes in the 65 patients who underwent non-invasive ambulatory blood pressure; however, effects of the ACE gene genotypes were observed on the derivatives of ambulatory blood pressure,:including sleeping blood pressure levels and circadian blood pressure rhythm (P < 0.05). We suggest that insertion/deletion polymorphism of the ACE gene should be associated with blood pressure and ambulatory blood pressure might be superior to casual blood pressure at the molecular level.