To determine the distribution of hepatitis C virus (HCV) genotypes in German isolates, nucleotide sequences of the viral nonstructural 5 (NS5) genome domains were analyzed in isolates from 107 chronically HCV-infected patients. Of these 107 patients, 46 (43.0% were infected with subtype 1a and 47 (43.9%) with subtype 1b. Six patients (5.6%) with a history of intravenous drug abuse were infected with subtype 3a. Eight patients (7.5%) who had acquired their HCV infection in Egypt carried subtype 4a. Forty-three of the 107 patients were treated with alpha-interferon. Of these 43 patients, 16 (37.2%) were infected with subtype 1a and 27 patients (62.8%) with subtype 1b. Three patients infected with HCV-subtype 1a (18.7%) and four patients infected with subtype 1b (14.8%) showed a sustained complete response after interferon therapy. The HCV genotype 1 with its subtypes 1a and 1b was the most common source of HCV infection in this group of patients. There was no significant difference in response to alpha-interferon treatment of HCV infection with the subtypes 1a or 1b.