Systemic mast cell disease associated with primary mediastinal germ cell tumor. The first known case of systemic mast cell disease associated with a germ cell tumor is reported. Six months after the complete remission of a primary mediastinal germ cell tumor treated by chemotherapy and resection, a young man had a series of episodes of hypotension and syncope and, a few months later, a gastric hemorrhage and coagulation disorder. The diagnosis of systemic mast cell disease was made in view of bone marrow and liver biopsies. A transient circulating heparin-like anticoagulant was noted. The occurrence in a short period of these two unusual conditions, given previous knowledge of hematologic disorders associated with germ cell tumors, suggests that the present association is not a coincidence. Systemic mast cell disease should be considered among the hematologic disorders associated with a germ cell tumor. 