The usefulness of urine cyclic guanosine 3'-5'monophosphate (GMP (cGMP) as a cancer marker in ovarian cancer has been studied in two independent centres using specimens from 81 women. Serial specimens from 60 patients were monitored up to 2 years after the start of chemotherapy. Urine cGMP levels were significantly higher in those patients with poorly differentiated tumours. Levels were not associated with the tumour stage, histological type or residual tumour remaining after surgery. Pretreatment levels were of no value for predicting response to therapy or patient survival. During therapy, however, levels did confirm clinical observations and if a level was elevated then there was a 80-90% chance that the patient was not responding to therapy. Using serial measurements, it was possible to predict the recurrence of tumour in 16/25 patients prior to any other clinical signs.