Background: Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors. Some cases of association with breast cancer have been described, but the possible etiological relationship between GIST and breast adenocarcinoma is unknown. Methods: We report on three patients with diagnoses of GIST and breast carcinoma at Memorial Sloan-Kettering Cancer Center. We review the literature regarding GIST-associated malignancies, with an emphasis on breast cancer. Results: We observed two cases of breast cancer patients diagnosed with GIST, which were detected during routine follow- up and staging procedures, respectively. A third case of breast ductal adenocarcinoma diagnosed in a patient with metastatic GIST with a long-term response to imatinib is also described. Clinical implications of an early diagnosis of otherwise generally asymptomatic neoplasms are described in this paper, together with the role of PET/CT in the staging and follow-up of both neoplasms. Conclusions: Reports of GIST associated with other neoplasms are becoming more common as patients live longer after diagnosis of other cancers, and as detection techniques improve in quality. It is difficult to exclude an incidental relationship between breast cancer and GIST, but the lack of an obvious increase in GIST in patients with familial breast cancer indicates that if there were a relationship, it would be through another genetic mechanism. Further studies are underway to clarify both the relationship of GIST with other cancers, as well as any possible role of c-kit in the development of breast adenocarcinoma.