Liver and plasma hepatitis C virus (HCV) variability was compared by E2 cloning and sequencing in three patients coinfected with HCV and human immunodeficiency virus (HIV) before and after interferon treatment and in three patients solely infected with HCV. The plasma and liver samples contained unique sequences. In the patients coinfected with HIV, accumulated random mutations produced mostly nonsynonymous substitutions in contrast to the reduced HCV genetic variability seen after treatment.