Aim: To evaluate changes in self-efficacy for self-management in young people with Type 1 diabetes participating in a 'Negotiated Telephone Support' (NTS) intervention developed using the principles of problem solving and social learning theory. Methods: One-year RCT with 79 young people (male 39; mean age ± SD 16.5 ± 3.2 years, duration 6.7 ± 4.4 years, HbA_1c 8.6 ± 1.5%) randomized into: Group 1 (control group), continued routine management, n = 28; Group 2, continued routine management with NTS, n = 25; Group 3, annual clinic with NTS, n = 26. Outcome measures: HbA_1c, self-efficacy, barriers to adherence, problem solving, and diabetes knowledge. Results: There were no differences between the groups at baseline. Participants in Groups 2 and 3 received an average of 16 telephone calls/year (range 5-19), median duration 9 min (2-30), with a median interval of 3 weeks (1-24) between calls. Significant correlations were found between age and average length of call (r = 0.44, P < 0.01) and frequency of contact (r = 0.36, P < 0.05). Social and school topics were discussed frequently. After 1 year, while the participants in the two intervention groups showed significant improvements in self-efficacy (P = 0.035), there was no difference in glycaemic control in the three groups. Barriers to insulin use adherence were a significant predictor of HbA_1c (P < 0.001) after controlling for baseline. Conclusions: NTS is an effective medium to deliver a simple theory-based psychological intervention to enhance self-efficacy for diabetes self-management. Reduced clinic attendance, combined with NTS, did not result in a deterioration of HbA_1c. Intensive personal support needs to be combined with intensive diabetes therapy to improve glycaemic control in this age group.