The serum tumour marker CA 125 is useful in the management of ovarian cancer, although it has its limitations. Approximately 85% of the ovarian cancer patients have an increased serum CA 125 at the start of treatment. There is a good correlation between the course of CA 125 and the clinical response of the tumour. Patients with an increasing CA 125 are found to have progressive disease in 97%. In these patients further examinations to document progression should be performed. A decrease in serum CA 125 corresponds in 87% of the patients with tumour regression. A normal serum CA 125, however, does not exclude tumour. More than 40% of the patients with a normal CA 125 still have microscopic or macroscopic tumour at second look surgery. On the other hand, an increased serum CA 125 corresponds in 90% of the patients with the presence of tumour or tumour progression shortly after the second look. The same holds for secondary debulking surgery. Patients with an increasing serum CA 125 before secondary debulking surgery have progressive disease at or shortly after surgery, even if an adequate tumour debulking has been performed. In these patients surgery should be omitted as long as no effective second line therapy is available. The course of serum CA 125 during the first 3 months of treatment is of prognostic value for response rate as well as time to progression and overall survival. Patients with a serum half-life of more than 20 days, or a CA 125 which is still high 3 months after the start of treatment, have a significantly lower response rate and progression-free survival. Whether it is possible to improve the prognosis of these patients by dose-intensification will have to be investigated in randomized trials. Serum CA 125 is not specific for ovarian cancer. High levels can also be found in patients with non-ovarian gynaecological and non-gynaecological tumours as well as patients with benign diseases and even in apparently healthy persons. In view of this aspecificity, serum CA 125 cannot be proposed for mass screening. For the same reason, serum CA 125 and the immunohistochemical staining with OC 125 are of limited value in the differential diagnosis between a primary ovarian tumour and metastatic disease in an ovary, as well as for differential diagnosis of pelvic tumours.