We studied the epidemiological, laboratory and histological characteristics of a group of patients with positive antibodies against hepatitis C virus (HCV) as determined by third-generation enzyme-linked immunosorbent assay (ELISA), and with indeterminate HCV antibody positivity as established by third-generation recombinant immunoblot assay (RIBA-3). The results obtained were compared with those recorded in a group of RIBA-3-positive patients. Both groups correspond to blood donors in whom the prevalence of hepatitis C is low. There were no statistically significant intergroup differences in mean age, or in the presence of infection risk factors. RNA positivity was much more frequent in the RIBA-positive group (71%vs 10%; P < 0.05), as was transaminase elevation during the 3 years of follow-up (54%vs 13%; P < 0.05). In 46% of the RIBA-indeterminate patients the liver biopsy proved normal, or only liver steatosis or minimal changes were detected, while 33% had persistent chronic hepatitis, and 21% showed active chronic hepatitis. A mean Knodell index score of 2.28 was recorded; 50% of the subjects showed no fibrosis, 46% grade 1 fibrosis (fibrous portal expansion), 4% grade 2 fibrosis (bridging fibrosis), and none grade 3 fibrosis (liver cirrhosis). In the RIBA-positive group, a greater percentage of patients had active chronic hepatitis, a greater Knodell index, and increased-grade fibrosis. It can be concluded that the RIBA-3-indeterminate group is epidemiologically similar to the RIBA-3-positive series, although with a lesser prevalence of laboratory test alterations, a lower viral replication index, and more likely to have benign disease - particularly in subjects without viral replication.