The presence of the Ca-125 antigen was tested in the serum of 46 patients with ovarian cancer in order to determine the prognostic value of preoperative levels and its usefulness for monitoring the clinical response in longitudinal studies; survival (S) and progression-free survival (PFS) were also evaluated. In our series, the specificity of the assay in normal subjects and in patients with benign gynecological diseases is 99.3 and 73.2% respectively, and the sensitivity is 91.9%. Preoperative Ca-125 levels are not correlated with S and PFS, whereas an advantage in S and PFS is clearly shown for patients in whom the marker level decreases after treatment. Serial determinations of Ca-125 serum levels provide a reliable test to assess response to therapy and to predict disease progression.