Imatinib mesylate is a drug that has been approved for treatment of advanced gastrointestinal stromal tumors (GISTs) and patients with leukemia such as chronic myeloid or Philadelphia chromosome-positive acute lymphoblastic. Although it has been described only in one patient with testicular hydrocele and gynecomastia in the literature, several cases of male gynecomastia have been reported with the use of imatinib mesylate in chronic myeloid leukemia (GML). Generally, male mastoplasia resolves after discontinuation of imatinib treatment. We report a 73-year-old male with metastatic GISTs who developed gynecomastia and unilateral testicular hydrocele while receiving imatinib mesylate. Nine months after commencing imatinib treatment, gynecomastia and testicular hydrocele were determined. Hormone analyses requested showed serum testosterone levels below and serum estrogen levels above normal limits. During the first month after discontinuing imatinib mesylate treatment, serum testosterone level was normal and there was a partial regression in gynecomastia and testicular hydrocele. To our knowledge, this is the second report of male gynecomastia following imatinib mesylate treatment of a patient with GIST. In conclusion, male patients who are to receive treatment with imatinib mesylate may be monitored for serum testosterone levels and for other reproductive hormone profiles before initiation of the treatment and their breasts may be examined during follow-up visits.