The Diabetes Prevention Trial Type 1 (DPT-1) has recruited relatives of patients with type 1 diabetes throughout the United States and Canada. Of the group screened before June 30, 2000, 71,148 initial screening samples of DPT-1 subjects were tested for GAD65 autoantibodies (GAA) and ICA512 (IA-2) autoantibodies (ICA512AA). Of 71,148 relatives screened, first-degree relatives (4.63%, n = 59,752) had a significantly higher prevalence of autoantibodies than did second-degree relatives (2.61%, n = 9,856) (P < 0.0001 for both autoantibodies). Among first-degree relatives, siblings (5.47%, n = 27,128) had a significantly higher prevalence of autoantibodies than did offspring (3.98%, n = 17,063) and parents (3.88%, n = 15,561) (P < 0.0001 for both autoantibodies). Among offspring, the offspring (n = 105) of both parents with diabetes had twice (8.57%) the prevalence of autoantibodies than did the offspring (n = 16,901) of a single diabetic parent (3.96%). Interestingly, the offspring (n = 8,777) of diabetic fathers had a significantly higher prevalence of autoantibodies than did the offspring (n = 8,124) of diabetic mothers, but only among those aged 10-30 years (P < 0.0001). We conclude that the prevalence of anti-islet cell autoantibodies is affected by multiple levels of relationship to the proband.