$Purpose$ $BRCA1/2$ mutations increase the risk of breast and prostate cancer in men. Common genetic variants modify cancer risks for female carriers of $BRCA1/2$ mutations. We investigated-for the first time to our knowledge-associations of common genetic variants with breast and prostate cancer risks for male carriers of $BRCA1/2$ mutations and implications for cancer risk prediction.

$Materials\; and\; Methods$ We genotyped 1,802 male carriers of $BRCA1/2$ mutations from the Consortium of Investigators of Modifiers of $BRCA1/2$ by using the custom Illumina OncoArray. We investigated the combined effects of established breast and prostate cancer susceptibility variants on cancer risks for male carriers of $BRCA1/2$ mutations by constructing weighted polygenic risk scores (PRSs) using published effect estimates as weights.

$Results$ In male carriers of $BRCA1/2$ mutations, PRS that was based on 88 female breast cancer susceptibility variants was associated with breast cancer risk (odds ratio per standard deviation of PRS, 1.36; 95% CI, 1.19 to 1.56; $P$ = 8.6 × 10$\mathrm{<mrow\; data-mjx-texclass="ORD">-6</mrow>}$)). Similarly, PRS that was based on 103 prostate cancer susceptibility variants was associated with prostate cancer risk (odds ratio per SD of PRS, 1.56; 95% CI, 1.35 to 1.81; $P$ = 3.2 × 10$\mathrm{<mrow\; data-mjx-texclass="ORD">-9</mrow>}$)). Large differences in absolute cancer risks were observed at the extremes of the PRS distribution. For example, prostate cancer risk by age 80 years at the 5th and 95th percentiles of the PRS varies from 7% to 26% for carriers of $BRCA1$ mutations and from 19% to 61% for carriers of $BRCA2$ mutations, respectively.

$Conclusion$ PRSs may provide informative cancer risk stratification for male carriers of $BRCA1/2$ mutations that might enable these men and their physicians to make informed decisions on the type and timing of breast and prostate cancer risk management.