In seven patients with insulin-dependent diabetes mellitus (IDDM) and 86 patients with non-insulin-dependent diabetes mellitus (NIDDM), serum anti-DNA antibody was measured by a semiquantitative radioimmunoassay (RIA) method. Prevalence of positive anti-DNA antibody (more than 20 U/mL) was five of seven in IDDM patients, 15 of 36 in NIDDM patients with insulin therapy, and seven of 50 in NIDDM patients without insulin therapy. None of normal subjects or patients with impaired glucose tolerance (IGT) showed positive anti-DNA antibody. The titer of anti-DNA antibody was higher in IDDM patients than in age-matched normal subjects (mean +/- SD; 22.1 +/- 15.3 v 6.5 +/- 2.2 U/mL, P less than .05). In patients with NIDDM, the antibody titer regardless of insulin treatment, was higher than in age-matched subjects with IGT (18.5 +/- 13.1 U/mL in NIDDM patients receiving insulin, 14.8 +/- 8.1 U/mL in NIDDM patients not receiving insulin, and 8.8 +/- 3.9 U/mL in IGT patients [P less than .001] for either of NIDDM groups v IGT). The titer of anti-DNA antibody was positively correlated with the duration of diabetes (r = .413, P less than .001) and with the postprandial blood glucose level (r = .311, P less than .01) in NIDDM patients when all of them were combined and analyzed as a group.(ABSTRACT TRUNCATED AT 250 WORDS)