Since it has been shown that the addition of interferon-a to ovarian cancer cells in vitro leads to an increase of CA125 in the cell culture supernatant it was the aim of our study to investigate whether the CA125 serum concentrations of patients with ovarian tumors could also be increased by injection of interferon-a in vivo. Nine patients with cystic ovarian tumors received 6 million IU interferon-α subcutaneously. CA125, CA15-3, and CEA were measured in defined time intervals during the following three days after interferon-α injection. In one patients with Tia NO MO ovarian cancer with normal serum levels of all markers before interferon-α application a massive increase of CA125 serum concentration was detected three days after injection. In the other 8 patients (5 benign tumors, 3 ovarian cancers) no marker increase was observed. In concordance with observations in cell cultures an increase of CA125 serum concentration after interferon-α injection was seen in one patient with early ovarian cancer and initially normal CA125 levels. If by the injection of interferon-α an increase of CAl25 could regularly be induced in ovarian cancer patients, earlier detection of ovarian cancer seems possible.