Insertion/deletion (I/D) polymorphism of the angiotensin converting enzyme (ACE) gene has been shown to be associated with various cardiovascular disorders in diabetic and non-diabetic patients. Its association with the development of non-insulin dependent diabetes mellitus (NIDDM) has been raised. This study was aimed to examine I/D polymorphism of ACE gene in healthy Thai subjects and patients with NIDDM. The I/D ACE genotypes were determined by polymerase chain reaction technique. Healthy unrelated subjects were 151 males and 147 females, 17-70 year old (mean +/- SD = 37.5 +/- 10.4). The unrelated diabetic patients were 42 males and 66 females, 20-79 years of age (mean +/- SD = 54.7 +/- 12.0). In healthy subjects, the ACE genotypes were DD 10.1 per cent, ID 39.2 per cent and II 50.7 per cent. Diabetic patients had similar distribution of ACE genotypes. The frequency of I and D alleles in diabetic patients was 0.69 and 0.31, similar to 0.70 and 0.30, respectively, in healthy subjects (p = 0.69). The frequency of I and D alleles in healthy Thai subjects was similar to the Japanese (I = 0.66 & D = 0.34) but different from Caucasians (I = 0.44-0.46 & D = 0.54-0.56). We conclude that I/D ACE gene polymorphisms may possess a racial difference. The similar frequency of both alleles in diabetic patients and healthy subjects suggests that there is no association between I/D polymorphism of ACE gene and diabetes mellitus in Thai individuals.