Multicentric Castleman's disease and Kaposi's sarcoma are more frequently observed in human immunodeficiency virus (HIV)-infected patients; however, 40-50% of the cases are HIV-negative. The present study reports the case of a 61-year-old man who presented with palpable masses in the axillary and right inguinal areas. The blood test results revealed increased serum erythrocyte sedimentation rate and C-reactive protein level, with negative serological markers, including for HIV. The patient was investigated using contrast-enhanced computed tomography (CT) and fluorine- 18fluorodeoxyglucose positron emission tomography (18F-FDG PET)/CT; the images revealed multiple enlarged and intensely hypermetabolic lymph nodes in the cervical, thoracic and abdominopelvic areas. Excisional biopsy and immunohistochemical analysis were performed, which confirmed the diagnosis of HIV-negative multicentric Castleman's disease coexisting with Kaposi's sarcoma. The patient received steroid therapy followed by chemotherapy. After 4 cycles of chemotherapy, the follow-up 18F-FDG PET/CT scan revealed nearly complete remission of the hypermetabolic malignant lesions of the neck, axilla and thoracoabdominal region.