A 42-year-old man with a remittent fever was found to have both para-aortic and hepatic tumors with generalized lymphadenopathy. The pathological findings from biopsy specimens from the para-aortic lymph node and hepatic tumor by laparotomy and from left supraclavicular lymphadenectomy showed undifferentiated carcinoma. However, the location of the primary lesion could not be determined. Chemotherapy temporarily reduced the size of the metastatic lymph nodes and the hepatic tumor and also suppressed the remittent fever. Fifteen months after onset, massive polyclonal hypergammapathy developed together with plasmacytosis (30% plasmacytes) in bone marrow, consisting of normal mature plasmacytes. Among cytokines, including interleukin (IL) 1 beta, IL-3, IL-4, IL-6, and tumor necrosis factor alpha, a high value of IL-6 was detected in the serum. Postmortem pathological examination showed the scirrhous type of gastric carcinoma with specific staining by polyclonal antibody against human IL-6. To our knowledge, this is the first report of a gastric carcinoma producing IL-6 associated with fever, hypergammaglobulinemia, and plasmacytosis in bone marrow.