A multicenter, NIH-sponsored study to determine if insulin administration prevents or delays the onset of IDDM in non-diabetic relatives of IDDM subjects at high risk for developing IDDM during the next five years. High-risk subjects will be randomized to low-dose insulin (twice daily by subcutaneous injection and once a year for 4 days by IV infusion) or a control group with no intervention. A 2-hour oral glucose tolerance test is performed twice a year. Studywide, 80,000 subjects will be screened to enroll 830 subjects.