• National Cancer Institute, Division of Cancer Epidemiology and Genetics, Bethesda, Maryland. Aremhe2@mail.nih.gov.
• National Cancer Institute, Division of Cancer Epidemiology and Genetics, Bethesda, Maryland.
• American Cancer Society, Atlanta, Georgia.
• Division of Research, Kaiser Permanente Northern California, Oakland, California.
• Department of Epidemiology and Community Medicine, University of Ottawa, Ottawa, Ontario, Canada.
• Cancer Research Center of Hawaii, Honolulu, Hawaii.
• International Agency for Research on Cancer, Lyon, France.
• Obstetrics and Gynecology Epidemiology Center, Department of Obstetrics, Gynecology and Reproductive Biology, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts. Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts. Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts.
• Divisions of Preventive Medicine and Aging, Brigham and Women's Hospital, Boston, Massachusetts.
• Cancer Registry of Norway, Oslo, Norway.

The association between leisure-time physical activity (LTPA) and male breast cancer risk is unclear. In the Male Breast Cancer Pooling Project, with 449 cases and 13,855 matched controls, we used logistic regression with study stratification to generate adjusted ORs and 95% confidence intervals (CI) for LTPA tertiles and male breast cancer risk. Compared with low LTPA, medium and high LTPA were not associated with male breast cancer risk (OR, 1.01; 95% CI, 0.79-1.29; 0.90, 0.69-1.18, respectively). In joint-effects analyses, compared with the referent of high body mass index (BMI; ≥25 kg/m(2))/low LTPA, neither medium nor high PA was associated with risk among high BMI men, but normal BMI men (<25 kg/m(2)) with low or medium LTPA were at a nonsignificant ∼16% reduced risk and those with high LTPA were at a 27% reduced risk (OR, 0.73; 95% CI, 0.50-1.07). Physical activity alone may not confer protection against male breast cancer risk.