BACKGROUND: Although psychological issues can interfere with diabetes care, the effectiveness of psychological treatments in improving diabetes outcomes is uncertain. OBJECTIVE: To determine whether motivational enhancement therapy with or without cognitive behavior therapy improves glycemic control in type 1 diabetes compared with usual care. DESIGN: Randomized, controlled trial. SETTING: 8 diabetes centers in London and Manchester, United Kingdom. PATIENTS: 344 adults with type 1 diabetes for longer than 2 years, with hemoglobin A(1c) levels of 8.2% to 15%, and without complications or severe comorbid disease. INTERVENTION: Nurse-delivered motivational enhancement therapy (4 sessions over 2 months), motivational enhancement therapy plus cognitive behavior therapy (12 sessions over 6 months), or usual care. MEASUREMENTS: 12-month change in hemoglobin A(1c) levels (primary outcome), hypoglycemic events, depression, quality of life, fear of hypoglycemia, diabetes self-care activities, and body mass index (secondary outcomes). RESULTS: In an analysis including all randomly assigned patients, the 12-month change in hemoglobin A(1c) levels compared with usual care was -0.46% (95% CI, -0.81% to -0.11%) in the motivational enhancement therapy plus cognitive behavior therapy group and -0.19% (CI, -0.53% to 0.16%) in the motivational enhancement therapy group alone. There was no evidence of treatment effects on secondary outcomes. LIMITATIONS: Of 1659 screened patients, only 507 were eligible and 344 participated. Data on the primary outcome were unavailable for 11.3% of the participants. Study design did not permit distinction of the additive effect of cognitive behavior therapy plus motivational enhancement therapy from the effect of greater intensity and duration of the combined intervention compared with the motivational enhancement therapy alone. CONCLUSION: Nurse-delivered motivational enhancement therapy and cognitive behavior therapy is feasible for adults with poorly controlled type 1 diabetes. Combined therapy results in modest 12-month improvement in hemoglobin A(1c) levels compared with usual care, but motivational enhancement therapy alone does not.