Objective: The intervention process was evaluated in a randomised clinical trial on the effectiveness of a behavioural Graded Exercise Therapy (GET) programme for patients with chronic shoulder complaints in primary care settings. This process evaluation was done for three reasons: first, to know whether the programme was performed as planned; second, to know whether or not this treatment is feasible in normal practice; third, to identify needs for improvement of the protocol to facilitate implication. Methods: We evaluated the extent to which the programme is being applied according to protocol, patient adherence to the programme and the suitability of the programme according to patients and physiotherapists. Twenty participating physiotherapists and 87 patients allocated to GET were asked to evaluate the intervention process. Furthermore, a sample of 10 patients, taken from the total number of 87 patients allocated to GET, was invited to take part in a focus group interview at the end of the study. We evaluated the intervention process using a treatment registration form filled in by the physiotherapists during treatment, evaluation forms filled in independently by physiotherapists and patients 26 weeks after the start of the treatment period, and information gathered during the focus group interview at the end of the study. Results: GET was applied according to protocol to an acceptable extent, and participating patients showed satisfactory adherence to the programme. Conclusions: Clinical effectiveness of GET in this trial is not influenced by flaws in the application of the programme or by lack of patient adherence. Practice implications: The programme could be improved in terms of the process of goal-setting, and the implementation of GET could be facilitated if the programme is embedded in a multidisciplinary approach in primary care.