OBJECTIVES: Mindfulness-based intervention has been receiving growing attention in cancer care. This study aimed to examine feasibility and to preliminary explore effectiveness of mindfulness-based cognitive therapy (MBCT) in Japanese breast cancer patients, and to explore possible modification of the program so that it fits better with this population. METHODS: Twelve participants with diagnosis of Stage I-III breast cancer received an eight session, weekly MBCT intervention in a group therapy format. The participants were followed up until 3 months after the completion of the program. RESULTS: All the participants completed the program with high attendance rate (mean number of attended sessions = 7.7). Significant improvement in anxiety (Hospital Anxiety and Depression Scale (HADS) - anxiety subscale; effect size Cohen's d = 0.88, P < 0.05), trauma-related psychological symptoms (Impact of Event Scale-revised; d = 0.64, P < 0.01) and quality of life (Functional Assessment of Cancer Therapy-Breast Cancer: FACT-B; d = 0.72, P < 0.01), and trend-level improvement in depression (HADS - depression subscale; d = 0.53, P = 0.054) were observed. Qualitative analyses suggested the program may be beneficial for alleviating fear of cancer recurrence and for increasing spiritual well-being. Some recommended modification of the program was indicated from the post-intervention interviews. CONCLUSIONS: Mindfulness-based cognitive therapy was well accepted by Japanese breast cancer patients and yielded favorable effect on their psychological status and quality of life. Further effectiveness study in a randomized-control design is warranted.