Follicular dendritic cell (FDC) sarcoma is an exceedingly rare neoplasm of unknown pathogenesis. A case of FDC sarcoma complicated by the hyaline-vascular type Castleman's disease occurring in a schizophrenic male is presented. Swelling of the left cervical lymph node appeared in a 44-year-old male schizophrenic who had been medicated with major tranquilizers for 20 years. He had had a history of cervical lymphadenopathy 14 years before, for which a diagnosis of hyaline-vascular type Castleman's disease had been made. The present specimen, obtained from the same site, was an enlarged lymph node heavily infiltrated with oval to spindle-shaped atypical cells but was uninvolved at the periphery. The infiltrating cells showed nodular or sheet-like growth, occasionally taking fascicular or storiform patterns. Follicular involvement was also common. Peculiarly, various amounts of small lymphocytes were intermingled with the neoplastic cells. The atypical cells expressed two FDC-specific antigens, DRC-1 and Ki-M4 antigen, together with a few other markers that are shared by FDC, including CD21 and HLA-DR. These findings clearly show the tumor to be FDC sarcoma. In addition, a peculiar fibro-hyalinous change in the lymph follicle, which is compatible with hyaline-vascular type Castleman's disease, was noted at the periphery of the lymph node where neoplastic cells had not infiltrated. Surprisingly, similar hyaline-vascular changes were observed in the previous biopsy taken 14 years ago. Meanwhile, Kaposi's sarcoma-associated herpesvirus, which is often identified from generalized Castleman's disease, was not identified in the present case by polymerase chain reaction study. Thus, this case is unique in two aspects: (i) the overlap of FDC sarcoma with hyaline-vascular type follicular changes; and (ii) its occurrence in a schizophrenic patient.