We traced 105 of 140 siblings of children with insulin-dependent diabetes (IDD) who had had oral glucose tolerance tests (OGTT) 10–12 yr earlier. Siblings with abnormal tests by screening criteria (8.3 mmol/L at 1 h, 7.2 at 2 h, N = 44) included all 6 who subsequently developed IDD after 3 mo to 7 yr (5.7% of entire group, 13.6% of abnormal screenees). The National Diabetes Data Group criterion for children (7.8 mmol/L at 2 h) identified 19 siblings, including 5 of the 6 who later developed IDD (26% of abnormals). Subsequent full 4-h OGTT, including analysis of insulin responses, did not improve predictability for subsequent IDD. Thus, siblings of IDD were identified at high risk (14–26%) or at low risk (0–1%) for subsequent IDD by a simple 2-h OGTT. The prolonged latency in the development of IDD indicates that, among siblings of IDD, this disorder may be already chronic for years by the time of clinical onset.