To evaluate the histological findings in patients with chronic hepatitis C and autoimmune features, liver tissue specimens from 60 patients were graded under code for individual features and composite patterns that denoted autoimmune, viral, combined autoimmune and viral, and nondiscriminative changes. Portal, interface, and acinar hepatitis in any combination with plasma cell infiltration connoted an autoimmune pattern that was associated with higher serum levels of gamma-globulin (2.4 +/- 0.2 g/dL vs. 1.7 +/- 0.1 g/dL; P = .0003) and immunoglobulin G (2,211 +/- 227 mg/dL vs. 1,508 +/- 83 mg/dL; P = .001) than patients with other patterns. Patients with the autoimmune pattern also had a greater frequency of cirrhosis (43% vs. 8%; P = .003), higher mean Knodell score (13.2 +/- 0.9 vs. 6.8 +/- 0.9; P < .0001), and a greater occurrence of high-titer smooth muscle antibodies (SMA) (13% vs. 0%; P = .05) than patients with other histological findings. HLA DR3 also occurred more frequently in these individuals than in other patients (48% vs. 15%; P = .01) and normal subjects (43% vs. 16%; P = .01). Patients with nondiscriminative patterns and interface hepatitis had clinical findings similar to those with autoimmune patterns, except for a lower mean serum level of gamma-globulin. We conclude that the composite histological pattern that resembles autoimmune hepatitis is associated with greater immunoreactivity, inflammatory activity, and disease severity than other patterns. Interface hepatitis may be the most important histological finding associated with these clinical manifestations.