To investigate the relationship among circulating cytokines, inflammation in the liver, and kind of response to interferon-alpha (IFN-alpha) in hepatitis C, we studied 63 consecutive patients (38 male, 25 female), treated with IFN for up to 1 year. Serum tumor necrosis factor-alpha (TNF-alpha) was measured at baseline and after 3 months of treatment. Transient (TR) or sustained response (SR) was observed in 29 and 16 patients, respectively. Baseline levels of TNF < or = 22 ng/L were observed in 69% of patients with SR, 55% of patients with TR, and 22% of nonresponders (p < 0.01). There was a significant correlation between baseline TNF levels and histologic grading score of hepatitis (p < 0.01). After 3 months of treatment, TNF levels >22 ng/L were observed in 63% of patients with SR, 69% of patients with TR, and 83% of nonresponders (p NS). Independent of the treatment outcome, TNF levels were lower at baseline and increased significantly with treatment in patients with lower histologic grading (p < 0.005). In conclusion, in patients with chronic hepatitis C, circulating TNF levels correlate with the degree of inflammation in the liver. Response to IFN is accompanied by an inflammatory response involving the release of TNF.