Primary sclerosing cholangitis (PSC) predisposes to the development of cholangiocarcinoma, a usually fatal complication that is difficult to diagnose. Serum concentrations of CA 19-9, a tumor-associated antigen, are frequently increased in patients with only cholangiocarcinoma. The aim of this study was to assess the value of an increased serum CA 19-9 level for the diagnosis of cholangiocarcinoma in patients with preexisting PSC. We analyzed serum samples from 9 patients with PSC and superimposed cholangiocarcinoma and from 28 patients with only PSC. Serum concentrations of CA 19-9 were measured in a blinded manner with use of an immunoradiometric assay. The serum CA 19-9 concentrations were increased in 8 of 9 patients (89) with PSC and cholangiocarcinoma (mean±SE, 391 ± 86 U/ml; range, 4 to 677), whereas they were increased in only 4 of 28 patients (14) with only PSC (mean ± SE, 61 ± 16 U/ml; range, 2 to 370). The sensitivity of a CA 19-9 value greater than 100 U/ml for cholangiocarcinoma in PSC was 89, and the specificity was 86. The measurement of serum concentrations of CA 19-9 is a promising test for detecting cholangiocarcinoma in patients with PSC.