We report a rare case demonstrating the relationship between secondary amyloidosis and uterine leiomyosarcoma. A 59-year-old female with high fever was referred to our hospital. Laboratory tests revealed increased white blood cells, eosinophilia and an accelerated erythrocyte sedimentation rate. Endoscopic examination of the stomach and colon revealed amyloid deposits in their mucosa. The patient showed no symptoms that suggested amyloidosis. No other organs or tissues were surveyed for amyloid deposition. Ga scintigraphy, computed tomography and magnetic resonance imaging suggested necrotic infectious leiomyoma of the uterus, which was considered to be the cause of the fever. The patient underwent total hysterectomy. The histological diagnosis of the mass revealed a low-grade uterine leiomyosarcoma with necrosis. Amyloid deposits in the gastric mucosa disappeared 1 year after the operation. In this case, amyloid deposition was detected by endoscopic biopsy before clinical manifestations. The deposition was reversible and was successfully treated. Thus, it is logical and useful to undertake endoscopic mucosa biopsy to check for amyloid deposition in patients with systemic inflammation, whose serum amyloid A protein level has been high for several months. In addition, peripheral eosinophilia was also detected in this case. Although eosinophilia associated with malignant tumor has been recognized, it is an uncommon occurrence.