Patients between the ages of 18 and 65 with the diagnosis of type 1 diabetes mellitus for at least 5 years may be eligible for this study. Candidates will be screened with a medical history and physical examination, blood tests, chest X-ray and tuberculin skin test, electrocardiogram and exercise test for heart function, abdominal ultrasound, psychological evaluation, and an arginine stimulated c-peptide test. The latter test determines if the patient is producing any insulin. Eligibility is restricted to patients who make no insulin at all.