Stored sera from 28 patients with inherited coagulation disorders who had developed non-A non-B hepatitis (NANBH) following a first exposure to clotting factor concentrates and 15 similar, but unmatched, patients who had received blood products but had normal transaminases on sequential testing were tested using the Ortho enzyme-linked immunosorbent assay (ELISA) anti-HCV assay. Twenty-seven of the 28 patients with NANBH were anti-HCV positive after exposure. In 10 of those in whom dates of first exposure and seroconversion were well-defined, the median time interval to NANBH was 4 weeks (range 1-7) and to anti-HCV seroconversion was 11 weeks (range 7.5-14.5). None of the 15 patients without NANBH developed anti-HCV. This first generation Ortho ELISA anti-HCV assay showed 96% sensitivity and 100% specificity and has potential use as an adjunct in the surveillance of new clotting factor products.