Serum CA125 is frequently used to monitor the treatment response and to detect early recurrence during the follow-up of patients who have epithelial ovarian tumors. In this report, the level of serum CA125 and urinary 5-hydroxyindole acetic acid of a patient with a carcinoid tumor which occurred in association with a mucinous adenocarcinoma of the ovary is described. The level of serum CA125 decreased despite the presence of a clinically progressive tumor. There was a rise of the 5-hydroxyindole acetic acid level 16 weeks before pelvic recurrence was detected clinically. This indicates that it was the neuroendocrine component, and not the mucinous component of the tumor, that was not responsive to the chemotherapy. The problem of using tumor markers to monitor patients with mixed tumors is discussed.