Psychosocial factors have been shown to play an important role in the development of chronic low back pain (LBP). In our recently completed cluster-randomized trial we found, however, no evidence of an effect of our minimal intervention strategy (MIS) aimed at psychosocial factors, over usual care (UC) in patients with (sub)acute LBP. To explore the reasons why, this paper presents an evaluation of the processes presumably underlying the effectiveness of MIS. General practitioner (GP) attitude was evaluated by the Pain Attitudes and Beliefs Scale and two additional questions. GP behaviour was evaluated by analysing treatment registration forms and patients' responses to items regarding treatment content. Patients also scored items on satisfaction and compliance. Modification of psychosocial measures was evaluated by analysing changes after 6 and 52 weeks on the Fear Avoidance and Beliefs Questionnaire, the Coping Strategies Questionnaire and the 4-Dimensional Symptom Questionnaire. A total of 60 GPs and 314 patients participated in the study. GPs in the MIS-group adopted a less biomedical orientated attitude than in the UC-group, but were only moderately successful in identification of psychosocial factors. Treatment contents as perceived by the patient and patient satisfaction differed significantly between both groups. Changes on psychosocial measures, however, did not differ between groups. The suboptimal identification of psychosocial factors in the MIS-group and the absence of a relevant impact on psychosocial factors may explain why MIS was not more effective than UC. ?? 2005 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.