Patients with chronic active hepatitis C show low lipoprotein(a) (Lp[a]) values. We studied the changes in Lp(a) levels caused by treatment with interferon in 24 patients (9 men and 15 women; mean age, 56.8 +/- 7.3 years) affected by chronic active hepatitis C. Fifteen healthy subjects (6 men and 9 women; mean age, 57.4 +/- 10.3 years) were used as controls. All of the patients with chronic hepatitis C were treated with intramuscular interferon, 3 million units 3 times per week for 6 months. These patients had lower baseline serum Lp(a) concentrations than the controls (4.8 +/- 3.8 mg/dL vs 13.4 +/- 10.3 mg/dL, respectively; P = 0.0007). A significant increase in Lp(a) levels (6.6 +/- 7.2 mg/dL; P = 0.05) occurred after 6 months of treatment in patients with chronic active hepatitis C. Only complete responders presented a significant increase in Lp(a) values (P = 0.01). We believe that increased Lp(a) levels represent an expression of improved liver functions.