The aim of the present study was to investigate persistence of the clinical effectiveness of a 5-weekly structured group intervention program for Japanese breast cancer patients. Each program is a series of five 90-min sessions, which include psycho-education, problem-solving, psychological support, relaxation training, and guided imagery. Fifty-seven patients with breast cancer participated in the program and the 47 who completed were analyzed. The results of the Profile of Mood States (POMS) scores were compared before, just after, and 6 months after the intervention. According to the change in POMS scores, the clinical effectiveness of a structured group intervention program persisted for 6 months for the patients who had no psychiatric diagnoses at entry. By contrast, this 5-weekly intervention program did not show persistent effects for patients who had a psychiatric diagnosis at entry. Therefore, additional intervention would be needed for patients with psychiatric diagnosis at entry.