The family connection of diabetes was examined from the clinical records of 3,372 subjects who were seen, as an out patient population, within the frame of a Regional Health Program in Taranto, South Italy. The family connection of diabetes resulted from a questionnaire in which the subjects had to give informations about their disease, if present, and degrees of relationship that were directly verified by us with the examination (clinic and laboratory) of relatives said to have diabetes. From the analysis of records, it emerged that 112 patients were affected by insulin-dependent-diabetes mellitus (IDDM): 54 of them were related with at least one subject suffering from noninsulin-dependent diabetes mellitus (NIDDM), 13 with at least one subject affected by IDDM and the remaining 45 did not show any family connection. The corresponding figures found in a group of healthy control subjects, matched to IDDM patients for age, sex and BMI, were 19, 2 and 84, respectively (p less than 0.001). 34 IDDM patients were related with a first degree of relationship (parents, sons, sibs) to diabetic subjects (IDDM or NIDDM), but only 4 controls showed such a degree (p less than 0.001). These results seem to indicate that patients with IDDM have an increased family history of NIDDM.