Researchers from The University of Tokyo in Japan conducted a meta-analysis of the effects of mindfulness-based stress reduction (MBSR) on the mental and physical health status of patients with various types of cancer.
Ten studies (randomized-controlled trials and observational studies) were determined to be eligible for meta-analysis.  Study results were categorized into mental and physical variables and Cohen's effect size d was computed for each category.

The investigators found that MBSR may indeed be helpful for the mental health of cancer patients (Cohen's effect size d=0.48); however, more research would be needed to show convincing evidence of the effects on physical health (Cohen's effect size d=0.18). They conclude that the results suggest that MBSR may improve cancer patients' psychosocial adjustment to their disease.

Citation: Ledesma D, Kumano H. Mindfulness-based stress reduction and cancer: a meta-analysis Psychooncology. 2009 Jun;18 (6): pages 571-9