A total of 42 Swedish patients with biopsy-proven chronic hepatitis C virus (HCV) infection were treated with a natural human leucocyte alpha-interferon (HuIFN-alpha-Le), Alfanative (BioNative AB, Umeå, Sweden) in an open uncontrolled study. Two patients were withdrawn from treatment within 2 weeks due to non-compliance and were omitted from further analysis, and 40 patients (17 females), mean age 39 years (range 24-71) completed the study. All patients were HCV RNA-positive in serum prior to treatment, with raised alanine aminotransferase (ALT) levels > 1.5 times the upper normal limit known for more than 6 months. Interferon was given at a dose of 3 MU t.i.w. for an intended 24 weeks and follow-up was a further 24 weeks after treatment. Biochemical non-responders were withdrawn from treatment within 12-16 weeks but continued follow-up. Overall 21/40 (52.5%) patients had a complete biochemical response with normal ALT levels at the end of treatment. Sustained response during follow-up was seen in 8 (20%) whereas 13 (32.5%) had a non-sustained response. At the end of treatment 23 (58%) patients had undetectable serum HCV RNA and 9 (23%) at follow-up. Patients with sustained, non-sustained and non-response had a mean pretreatment HCV RNA level of 3.2 x 10(5), 2.5 x 10(6) and 3.2 x 10(6) genomes/ml, respectively, differences that did not reach statistical significance. Of the patients 3, 9, 10 and 14 had genotype 1b, 3a, 1a, and 2b, respectively, and 4 had mixed genotypes. Of the 23 patients with genotype 2b or 3a, 7 had a sustained response vs. none of the 13 patients with genotype 1a or 1b (p = 0.03). No patients with cirrhosis had a sustained response whereas 4/18 with chronic persistent and 4/18 with chronic active hepatitis had such a response. It is concluded that some 50% of patients treated with HuIFN-alpha-Le responded with normalisation of ALT levels but that only 20% had a durable response 24 weeks post-treatment, and that patients with genotypes 3a or 2b seem to respond better than patients with other genotypes.