Cytokeratins are polypeptides that constitute a subclass of intermediate filaments in epithelial cells. The aim of the present study was to evaluate the clinical usefulness of the serum evaluation of M3/M21 in patients with ovarian cancer. This retrospective study comprises 75 patients suffering from ovarian cancer FIGO stages Ia-III. M3/M21 reached a sensitivity of 78%, a specificity of 85%, a PPV of 89% and a NPV of 83% using a cut-off level of 45 U 1(-1). Forty-four women developed recurrent disease after complete remission during the observation period. M3/M21 showed lead time effects in 19 patients, ranging from 2 to 8 months (median 3.2 months). Elevated M3/M21 serum levels before therapy were associated with a poor overall survival (log-rank test, P = 0.02). Considering these preliminary results, the value of M3/M21 as a serum tumour marker, i.e. to evaluate the tumour burden, seems promising.