We studied interleukin-6 production in 4 human renal cell carcinoma cell lines and measured the serum level in 71 patients with renal cell carcinoma, thus, clarifying a relationship between interleukin-6 secretion and an occurrence of the paraneoplastic syndrome in the carcinoma. Interleukin-6 was produced by 3 cell lines and detected in 25% of the patients. The level of interleukin-6 did not directly correlate with tumor volume and the differentiation grade of the carcinoma. However, the positive rate increased with progression of the stage. The serum level affected the 5-year survival of patients without distant metastasis. When serum interleukin-6 was elevated patients had a significantly higher frequency of unexplained fever and an elevation of acute phase proteins. These results suggest that some renal cell carcinomas can produce interleukin-6 and this cytokine is responsible for several paraneoplastic syndromes in the carcinoma.