The Schwabing Insulin Prophylaxis Trial is a randomised, controlled pilot study designed to examine whether insulin therapy can delay or prevent the clinical onset of Type I diabetes in high risk first degree relatives of people with the disease. First degree relatives of patients with Type I diabetes, who were aged 4 years or more, had an islet cell antibody (ICA) value more than 20 Junevile Diabetes Foundation Units (JDF-U), a reduced first phase insulin response (FPI) to an i. v. glucose tolerance test less than the 5th centile, and a normal oral glucose tolerance test were eligible for the trial. Between January 1989 and October 1995, 1736 relatives of patients with Type I diabetes were screened for ICA. We identified 64 cases (3.7 %) with ICA values more than 20 JDF-U. Of ICA positive relatives, 17 (27 %) had a low FPI and were eligible for enrolment. Of these 14 agreed to participate, of whom 7 were randomised to the treatment group and 7 to the control group. In the treatment group, human insulin was administered i. v. by continuous infusion for 7 days, followed by daily s. c. injections for 6 months. Intravenous insulin infusions were repeated every 12 months. In the treatment group 3 of the 7 individuals (follow-up from time of eligibility: 2.3 to 7.1 years) and in the control group 6 of the 7 untreated individuals (1.7 to 7.1 years) developed clinical diabetes. Life table analysis showed that clinical onset of Type I diabetes was delayed in insulin-treated subjects compared with control subjects (means ± SEM diabetes-free survival: 5.0 ± 0.9 years vs 2.3 ± 0.7 years, p < 0.03). Insulin levels after i. v. glucose increased in the first year of intervention therapy. Titres of ICA, and antibodies to glutamic acid decarboxylase, and tyrosine phosphatase-like protein IA2 remained unchanged. These data suggest that insulin prophylaxis can delay the onset of overt diabetes in high risk relatives. This is encouraging in view of 1) the continuing American Diabetes Prevention Trial, which is currently testing the effect of parenteral insulin in a large nation-wide study and 2) the initiation of pilot trials to determine whether new antigen-specific intervention is more effective in delaying the clinical onset of Type I diabetes. [Diabetologia (1998) 41: 536–541]