The aim of this study was to update our previous meta-analysis of interferon (IFN) in the treatment of hepatitis C and to analyse new factors, namely, HCV RNA end-point, patients with cirrhosis and patients with normal ALT. We use the Der Simonian and Laird method, with heterogeneity and sensitivity analyses. Seventy-six randomized control trials (RCTs) in naive patients were found but we focused our analysis on 59 RCTs with chronic hepatitis C (26 vs. controls and 33 comparing different regimens) and on seven RCTs in acute hepatitis. Interferon-alpha (IFN-alpha) at 3 MU thrice weekly (TIW) for 12 months exhibited 39% of virological end-of-treatment response (ETR) and 17% of virological sustained response (SR), respectively, vs. 1% and 3% in untreated controls (all P < 0.001). There was a significant dose effect (in favour of 6 vs. 3 MU TIW): the virological SR at 6 months were 35% in the 6 MU group (95% CI: 24-47) and 16% in the 3 MU group (95% CI: 8-27) and were at 12 months 43% in the 6 MU group (95%CI: 31-56) and 25% in the 3 MU group (95% CI: 16-37). There was a significant duration effect (12 vs. 6 months) upon the virological SR rate both at 3 and 6 MU: 3 MU provided 14% of virological SR (95% CI: 11-19) in the 12 months group vs. 7% (95% CI: 5-11) in the 6 months group and 6 MU provided 22% (95% CI: 17-29) and 16% (95% CI: 11-22) virological SR in the 12 and 6 months groups, respectively. Cirrhotic treated patients had 17% of virological SR (95 CI: 9-24%; P < 0.001) vs. 0% in controls and provided a 20% reduction rate (95 CI: -2% to -37%, P=0.03) in hepatocellular carcinoma incidence. In acute hepatitis C, a 3-month treatment with IFN-alpha showed significant efficacy vs. controls upon the virological SR rate (32% vs. 4%, P < 0.001). In conclusion, we confirm the dose and duration effect of IFN in chronic hepatitis C, and the efficacy of IFN-alpha in the treatment of acute hepatitis and in cirrhotic patients.