ObjectivesThe aim of this study was to evaluate the effectiveness of brief psychosocial support for patients with cancer and their relatives regarding their mental health.DesignQuasi-experimental controlled trial with measurements at three time points (baseline, after 2 weeks and after 12 weeks).SettingThe intervention group (IG) was recruited at two cancer counselling centres in Germany. The control group (CG) included patients with cancer or relatives who did not seek support.ParticipantsIn total, n=885 participants were recruited and n=459 were eligible for the analysis (IG, n=264; CG, n=195).InterventionOne to two psychosocial support sessions (approximately hour) provided by a psycho-oncologist or social worker.Primary and secondary outcome measuresThe primary outcome was distress. The secondary outcomes were anxiety and depressive symptoms, well-being, cancer-specific and generic quality of life (QoL), self-efficacy and fatigue.ResultsThe linear mixed model analysis showed significant differences between IG and CG at follow-up for distress (d=0.36), p=0.001), depressive (d=0.22), p=0.005) and anxiety symptoms (d=0.22), p=0.003), well-being (d=0.26, p=0.002), QoL (QoL mental; d=0.26, p=0.003), self-efficacy (d=0.21, p=0.011) and QoL (global; d=0.27, p=0.009). The changes were not significant for QoL (physical; d=0.04, p=0.618), cancer-specific QoL (symptoms; d=0.13, p=0.093), cancer-specific QoL (functional; d=0.08, p=0.274) and fatigue (d=0.04, p=0.643).ConclusionThe results suggest that brief psychosocial support is associated with the improvement of mental health of patients with cancer and their relatives after 3 months.Trial registration numberDRKS00015516.