PURPOSE: The purpose of this study was to examine reasons for nonparticipation and drop out in a diabetes self-management intervention. METHODS: A total of 468 recently screen-detected patients, receiving usual care or intensive pharmacological treatment, were invited and randomized into either a control or intervention condition, consisting of a brief self-management course. A nonresponse survey was conducted, and participants, nonparticipants, and dropouts were compared on sociodemographic variables, diabetes attitudes, and self-care. RESULTS: A total of 227 patients consented and were allocated to the control (n=108) or intervention group (n=119). Two hundred forty-one patients declined participation, 41 dropped out, and 78 completed the intervention. Major reasons for refusal and drop out were hesitancy toward research and practical barriers. Nonparticipants were less educated and reported higher self-management, while participation also varied by treatment and disease duration: intensively treated patients were more likely to participate in their first year, and usual-care patients participated more often 2 to 3 years after diagnosis. Dropouts had a lower education level but did not differ on any other measure. CONCLUSION: Participants, nonparticipants, and dropouts did not differ in their attitudes toward diabetes, but the intervention did attract patients with lower self-care. Variations in participation by treatment and disease duration suggest that patients prefer self-management interventions at different times depending on their medical treatment. Finally, education appears to be the most important factor determining participation. Alternative strategies are needed to attract and retain patients with low education.