OBJECTIVE: To assess in a randomized crossover trial the efficacy of continuous subcutaneous insulin infusion in improving glycemic control and health-related quality of life in type 1 diabetic patients with long-standing poor glycemic control. RESEARCH DESIGN AND METHODS: A total of 79 patients in 11 Dutch centers were randomized to 16 weeks of continuous subcutaneous insulin infusion followed by 16 weeks intensive injection therapy or the reverse order. Glycemic control was assessed by HbA(1c), self-reported hypoglycemic events, and blood glucose memory meter read outs. Changes in quality of life were assessed by self-report questionnaires administered at baseline and 16 weeks. RESULTS: As the drop-out rate after crossover was high (17 of 79 patients [22%]), we analyzed the trial as a parallel clinical trial, using data of the first half of the crossover phase only. At 16 weeks, mean HbA(1c) was 0.84% (95% CI -1.31 to -0.36) lower in the continuous subcutaneous insulin infusion group compared with the insulin injection group (P = 0.002). Stability of blood glucose self-measurement values, expressed as SD of the nine-point blood glucose profiles, improved in the insulin pump group by 29.3 +/- 41.1 vs. 8.2 +/- 36.5% in the injection group (P = 0.039). The number of mild hypoglycemic episodes per patient-week was 0.99 (95% CI 0.11-1.87) higher in the insulin pump group (P = 0.028). Weight gain was similar in both groups. Scores on the Short-Form 36-Item subscales 'general health' and 'mental health' improved in the continuous subcutaneous insulin infusion group, compared with stable values in the injection group (P = 0.048 and 0.050, respectively). CONCLUSIONS: Continuous subcutaneous insulin infusion improves glycemic control and some aspects of health-related quality of life in patients with a history of long-term poor glycemic control.