The age of onset of diabetes and the type of diabetes were examined in 1408 Japanese patients who were initially diagnosed as having diabetes under the age of 30 and were registered in our Diabetes Center between 1980 and 1989. Of the 1408 patients, 538 (38.2%) had insulin-dependent diabetes mellitus (IDDM) (male/female ratio of 2:3), and 870 (61.8%) had non-insulin-dependent diabetes mellitus (NIDDM) (male/female ratio of 5:4). There were significant differences of the sex ratio in both IDDM and NIDDM. The age at which the numbers in both the IDDM and NIDDM groups were almost equal was 13-14 (26 for IDDM and 23 for NIDDM at 13; 28 for IDDM and 30 for NIDDM at 14). A total of 58% of IDDM patients (22% of all patients) and only 6% of NIDDM patients (4% of all patients) were diagnosed under the age of 14 (P less than 0.01). Of the patients with IDDM, 42% (16% of all patients) were diagnosed over the age of 14, as were 94% of NIDDM (58% of all patients). The percentage of NIDDM cases increased even more over the age of 28, and no NIDDM patients developed diabetes under the age of 9.