Background: A program supporting the initiation of insulin therapy in primary care was introduced in Belgium, as part of a larger quality improvement project on diabetes care. This paper reports on a study exploring factors influencing the engagement of general practitioners (GPs) in insulin therapy initiation (research question 1) and exploring factors relevant for future program development (research question 2). Methods: We have used semi-structured interviews to answer the first research question: two focus group interviews with GPs who had at least one patient in the insulin initiation program and 20 one-to-one interviews with GPs who were not regular users of the overall support program in the region. To explore factors relevant for future program development, the data from the GPs were triangulated with data obtained from individual interviews with patients (n=10), the diabetes nurse educator (DNE) and the specialist involved in the program, and data extracted from meeting reports evaluating the insulin initiation support program. Results: We found differences between GPs engaged and those not engaged in insulin initiation in attitude, subjective norm and perceived behavioural control regarding insulin initiation. In general the support program was evaluated in a positive way by users of the program. Some aspects need further consideration: job boundaries between the DNE and GPs, job boundaries between GPs and specialists, protocol adherence and limited case load. Conclusion: The study shows that the transition of insulin initiation from secondary care to the primary care setting is a challenge. Although a support program addressing known barriers to insulin initiation was provided, a substantial number of GPs were reluctant to engage in this aspect of care. Important issues for future program development are: an interdisciplinary approach to job clarification, a dynamic approach to the integration of expertise in primary care and feedback on protocol adherence. Trial registration: ClinicalTrials.gov ...