OBJECTIVE -To determine the role of early behavioral and psychological factors on later outcomes in young adults with childhood-or adolescent-onset type 1 diabetes.RESEARCH DESIGN AND METHODS -We conducted a longitudinal cohort study of patients recruited from the register of the young adult outpatient diabetes clinic, Oxford, U.K. A total of 113 individuals (51 male subjects) aged 17-25 years completed assessments, and 87 (77%) were reinterviewed as older adults (aged 28 -37 years). Longitudinal assessments were made of glycemic control (HbA 1c ) and complications. Psychological state at baseline was assessed using the Present State Examination and self-report Symptom Checklist, with corresponding interview schedules administered at follow-up.RESULTS -There was no significant improvement between baseline and follow-up in mean HbA 1c levels (8.5 vs. 8.6% in men, 9.3 vs. 8.7% in women). The proportion of individuals with serious complications (preproliferative or laser-treated retinopathy, proteinuria or more severe renal disease, peripheral neuropathy, and autonomic neuropathy) increased from 3-37% during the 11-year period. Women were more likely than men to have multiple complications (23 vs. 6%, difference 17%, 95% CI 4 -29%, P ϭ 0.02). Psychiatric disorders increased from 16 to 28% (20% in men, 36% in women at follow-up, difference NS), and 8% had psychiatric disorders at both assessments. Baseline psychiatric symptom scores predicted follow-up scores ( ϭ 0.32, SE [] 0.12, P ϭ 0.008, 95% CI 0.09 -0.56) and recurrent admissions with diabetic ketoacidosis (odds ratio 9.1, 95% CI 2.9 -28.6, P Ͻ 0.0001).CONCLUSIONS -The clinical and psychiatric outcome in this cohort was poor. Psychiatric symptoms in later adolescence and young adulthood appeared to predict later psychiatric problems.