A group of South African Indians with NIDDM participated in a study to evaluate the frequency of positive family histories of the disease and to determine the relative contribution of maternal or paternal genetic determinants. Information was elicited by means of an interview and recorded. Of the 1098 diabetic subjects studied 70% gave a positive family history of a first degree relative suffering from NIDDM. Three-generation transmission was recorded in 5.3% of the subjects. A significantly greater proportion of probands (40%) had a mother with NIDDM than those with a father (26%). A positive family history in an offspring was more common in female probands (10.6%) than males (5.5%). Twice as many probands with 3 generation transmission had a maternal grandmother suffering from NIDDM (2.5%) compared with those who had a paternal grandmother afflicted (1.2%) (P < 0.05), whereas the frequencies in the maternal (0.9%) and paternal (0.8%) grandfathers were similar. This study has highlighted, not only the high prevalence of a positive family history in South African Indians with NIDDM, but also a stronger maternal contribution to the putative gene responsible for the disease.