BACKGROUND: The etiology of male breast cancer is poorly understood, partly 
because of its relative rarity. Although genetic factors are involved, less is 
known regarding the role of anthropometric and hormonally related risk factors.
METHODS: In the Male Breast Cancer Pooling Project, a consortium of 11 
case-control and 10 cohort investigations involving 2405 case patients (n = 1190 
from case-control and n = 1215 from cohort studies) and 52013 control subjects, 
individual participant data were harmonized and pooled. Unconditional logistic 
regression generated study design-specific (case-control/cohort) odds ratios 
(ORs) and 95% confidence intervals (CIs), with exposure estimates combined using 
fixed effects meta-analysis. All statistical tests were two-sided.
RESULTS: Risk was statistically significantly associated with weight 
(highest/lowest tertile: OR = 1.36; 95% CI = 1.18 to 1.57), height (OR = 1.18; 
95% CI = 1.01 to 1.38), and body mass index (BMI; OR = 1.30; 95% CI = 1.12 to 
1.51), with evidence that recent rather than distant BMI was the strongest 
predictor. Klinefelter syndrome (OR = 24.7; 95% CI = 8.94 to 68.4) and 
gynecomastia (OR = 9.78; 95% CI = 7.52 to 12.7) were also statistically 
significantly associated with risk, relations that were independent of BMI. 
Diabetes also emerged as an independent risk factor (OR = 1.19; 95% CI = 1.04 to 
1.37). There were also suggestive relations with cryptorchidism (OR = 2.18; 95% 
CI = 0.96 to 4.94) and orchitis (OR = 1.43; 95% CI = 1.02 to 1.99). Although age 
at onset of puberty and histories of infertility were unrelated to risk, never 
having had children was statistically significantly related (OR = 1.29; 95% CI = 
1.01 to 1.66). Among individuals diagnosed at older ages, a history of fractures 
was statistically significantly related (OR = 1.41; 95% CI = 1.07 to 1.86).
CONCLUSIONS: Consistent findings across case-control and cohort investigations, 
complemented by pooled analyses, indicated important roles for anthropometric 
and hormonal risk factors in the etiology of male breast cancer. Further 
investigation should focus on potential roles of endogenous hormones.