OBJECTIVES: To clarify the details of psychosocial health status, the effects and related costs of psychosocial group intervention in post-breast cancer surgery patients. METHODS: Structured psychosocial group intervention (90 min/week × 5 sessions) was conducted for 66 patients aged 20–79 years who were receiving breast cancer treatment at 3 hospitals in Japan (intervention group). The intervention was conducted 2 weeks–3 months after radical surgery. HRQOL and psychosocial function were investigated prospectively for outcome. The survey included 1) EORTC QLQ-C30 for HRQOL, and 2) POMS, MAC scale, and a characteristic self-efficacy scale for psychosocial function. It was conducted at registration, week 4 (group therapy conclusion), and month 6. Prior to the intervention study, a group that fulfilled the same eligibility criteria but did not receive psychosocial group intervention was studied (n=116; non-intervention group). The same outcomes were measured and compared using multivariate analysis adjusted for disease stage, chemotherapy, and hospital. Direct medical costs were obtained from receipt data, and information on travel expenses, direct nonmedical costs, and indirect costs (productivity costs) were collected simultaneously with the HRQOL survey. RESULTS: In HRQOL and psychosocial function, an intervention effect was seen in fatalism (MAC_F) only. In a subgroup analysis by hospital, effects of cognitive function were seen in HRQOL (EORTC_CF), tension-anxiety (POMS_TA), helplessness/hopelessness (MAC_H), and fatalism (MAC_F). However, these effects were not seen at 6 months. Total medical costs including indirect costs during the 6 months were ¥840,000 for the intervention group and ¥750,000 for the non-intervention group. CONCLUSIONS: Structured psychosocial group intervention for post-breast cancer surgery patients had a uniform effect in improving HRQOL and psychosocial function. It is significant that a benefit was obtained in the early postoperative period when HRQOL is most easily damaged. Medical costs did not differ significantly between the groups.