The prevalence of both NIDDM and IGT vary considerably within and between developing island populations of the Pacific and Indian Ocean regions. Longitudinal data have been collected recently in a number of these populations, allowing incidence rates to be compared. The incidence of NIDDM in adults ranged from a low of 1.2/1000 person-years (p.y.) in peri-urban and rural Papua New Guinea (PNG) Highlanders to 22.5/1000 p.y. in Micronesian Nauruans and 24.0/1000 p.y. in the rural Wanigelas of coastal PNG. Intermediate rates were observed in Polynesian Western Samoans (16.6 and 5.7/1000 p.y. in urban and rural areas, respectively) and ethnically diverse Mauritians: Asian Indians (15.8), African-origin Creoles (12.2), and Chinese (10.4/1000 p.y.). When stratified by age and body mass index (BMI), incidence in Wanigelas exceeded rates observed in Pima Indians, and rates in Mauritians were higher than those of Nauruans. For subjects with IGT at baseline, rates of conversion to NIDDM ranged from 19.0 to 102.6/1000 p.y. Particularly after stratifying for age and body mass index, it was apparent that there was less variation between populations in rates of decompensation from IGT than was observed for total incidence. The relative risk of conversion to NIDDM for IGT versus normal subjects ranged from 2.1 in urban Samoans to 7.6 in Nauruans, but most estimates exceeded 5.