BACKGROUND: Common mental health problems are mainly treated in primary care settings and collaboration with mental health services is needed. Prior to re-organisation of the mental health care offer in a geographical area, a study was organized: 1) to evaluate GPs' opinions on their day-to-day practice with Patients with Mental Health Problems (PMHP) and on relationships with Mental Health Professionals (MHPro); 2) to identify factors associated with perceived need for collaboration with MHPro and with actual collaboration. METHODS: All GPs in the South Yvelines area in France (n = 492) were informed of the implementation of a local mental health program. GPs interested in taking part (n = 180) were invited to complete a satisfaction questionnaire on their practice in the field of Mental Health and to include prospectively all PMHP consultants over an 8-day period (n = 1519). For each PMHP, data was collected on demographic and clinical profile, and on needs (met v. unmet) for collaboration with MHPro. RESULTS: A majority of GPs rated PMHP as requiring more care (83.4%), more time (92.3%), more frequent consultations (64.0%) and as being more difficult to refer (87.7%) than other patients. A minority of GPs had a satisfactory relationship with private psychiatrists (49.5%), public psychiatrists (35%) and social workers (27.8%). 53.9% had a less satisfactory relationship with MHPro than with other physicians. Needs for collaboration with a MHPro were more often felt in caring for PMHP who were young, not in employment, with mental health problems lasting for more than one year, with a history of psychiatric hospitalization, and showing reluctance to talk of psychological problems and to consult a MHPro. Needs for collaboration were more often met among PMHP with past psychiatric consultation or hospitalization and when the patient was not reluctant to consult a MHPro. Where needs were not met, GP would opt for the classic procedure of mental health referral for only 31.3% of their PMHP. CONCLUSION: GPs need targeted collaboration with MHPro to support their management of PMHP, whom they are willing to care for without systematic referral to specialists as the major therapeutic option.