Serial serum CA 125 levels were measured before definitive surgery and during chemotherapy for 12 months or more in 64 patients with ovarian cancer. In the 42 patients who had a complete clinical remission and thus were subjected to a second-look laparotomy, an absence of disease was not predicted by patterns of CA 125 levels. Whilst rising or persistently high levels indicated the presence of tumour in 92% of patients, declining levels to negative predicted the absence of tumour in only 50%. Although the majority of these patients showed microscopic foci or a tumour mass less than 1 cm, 3 patients had a larger amount of disease. In the follow-up of 49 patients, the accuracy of prediction of a good outcome was better than that of a poor outcome on the basis of CA 125 patterns, with rates of 92% and 79%, respectively. Our findings indicate that CA 125 lacks sensitivity in detecting small tumour masses (less than 1 cm dia.) but rising or persistently high levels suggest a strong likelihood of a residual tumour to be found at a second-look laparotomy.