Rationale:Mastocytosis is a group of rare neoplastic diseases characterized by monoclonal proliferation of mast cells in the skin or other tissues and organs, including cutaneous mastocytosis and systemic mastocytosis (SM). Mastocytosis can also occur in the gastrointestinal tract, mostly manifested as increased mast cells dispersed in various layers of the intestinal wall; a few may present as polypoid nodules, but rarely as soft tissue mass formation. Pulmonary fungal infections mostly occur in patients with low immune function and have not been reported in the literature as the initial manifestation in patients with mastocytosis. In this case report, we present the enhanced computed tomography (CT), fluorodeoxyglucose (FDG) positron emission tomography/CT, and colonoscopy findings of a pathologically confirmed patient with aggressive SM of the colon and lymph nodes and extensive fungal infection of both lungs.Patient concerns:A 55-year-old female patient visited our hospital because of repeated cough for more than half a month. Laboratory tests revealed a significantly high CA125 serum level. Chest CT showed multiple plaques and patchy high-density shadows in both lungs, and a small amount of ascites was observed in the lower-level image. Abdominal CT revealed a soft tissue mass with an ill-defined boundary in the lower ascending colon. Whole-body positron emission tomography/CT images showed multiple nodular and patchy density-increasing lesions with significantly increased FDG uptake in both lungs. The wall of the ascending colon in the lower segment was significantly thickened with soft tissue mass formation, and retroperitoneal lymph node enlargement was accompanied by increased uptake of FDG. Colonoscopy revealed a soft tissue mass at the base of the cecum.Diagnosis:Colonoscopic biopsy was performed and the specimen was diagnosed with mastocytosis. At the same time, a puncture biopsy was also performed on the patient’s lung lesions, and pulmonary cryptococcosis was considered a pathological diagnosis.Interventions:The patient was in remission after repeated treatment with imatinib and prednisone for 8 months.Outcomes:In the ninth month, the patient suddenly died of a cerebral hemorrhage.Lessons:Gastrointestinal involvement due to aggressive SM presents with nonspecific symptoms and different endoscopic and radiologic findings. This is the first report of a single patient with colon SM, retroperitoneal lymph node SM, and extensive fungal infection in both lungs.