Familial risks of breast cancer (BC) are well established but whether BC clusters with other, i.e. discordant, cancers is less certain but of interest for the identification of common genetic and possible environmental factors contributing to a general cancer susceptibility. We apply a novel approach to search for familial associations of BC with other (discordant) cancers based on the Swedish Family‐Cancer Database. Relative risks (RRs) were calculated for BC in families with increasing numbers of patients with discordant cancer X, and conversely, familial RRs for cancer X in families with increasing numbers of BC patients. Joint p‐values were calculated from independent analyses. The total number of familial BCs was 12,266, 14.9% with one first‐degree relative with BC and 1.2% with at least 2 affected relatives. Ovarian and prostate cancers showed the strongest associations with BC (p‐value <10−11). The p‐value for melanoma was <10−6, for stomach and male colorectal cancer <2.5 × 10−6, for cancer of unknown primary <2.5 × 10−5 and for lung cancer <5 × 10−5. Significance level <5 × 10−4 was reached with pancreatic cancer. The remaining associations (p < 0.0025) included thyroid, endometrial, testicular, eye cancers (uveal melanoma), nervous system and endocrine tumors and non‐Hodgkin lymphoma. The RR for BC increased by increasing numbers of patients with any cancer in family members and it reached 1.62 when three or more family members were affected. The results suggest that BC shares susceptibility with a number of other cancers. This might alert genetic counselors and challenge approaches for gene and gene–environment identification. What's new? It is well known that first‐degree relatives of breast‐cancer (BC) patients have also an increased risk of breast and ovarian cancer. But what about other, seemingly unrelated types of cancer? In our large study, the authors found that families with BC have also significantly higher rates of several other cancers, including melanoma, stomach cancer and leukemia. The converse was also true – families with other cancers have also increased rates of BC. These results suggest that BC shares genetic or environmental susceptibility factors with other types of cancer.