Cyfra 21-1 has been reported to be an effective tumor marker for epithelial carcinomas. The purpose of this investigation was to determine its value in evaluating response to treatment in patients with carcinoma of the cervix. Cyfra 21-1 levels were measured by immunoassay in the serum of 55 untreated patients with cervical cancer; a second sample was obtained in all of them after conventional treatment for association with clinical response. Pre-therapy levels were elevated in only 45% (25 of 55) of the patients, with a slight tendency to increase according to clinical stage: 33% (5/15) in stage I, 36% (8/22) in stage II and 67% (12/18) in stage III. In regards to association with response to therapy, and including patients with either normal or elevated pretreatment values, 46% (19/41) of women with a complete clinical response either persisted with or developed elevated levels after treatment completion. All 14 patients with persistent disease after therapy continued to have or developed elevated values. No patient with persistent disease had normal values after therapy, and all patients with negative values after treatment were truly complete clinical responders. The results of our study suggest that the test has a low sensitivity therefore and, despite our findings, a negative level after treatment may not be a safe indicator of disease-free status. On the other hand, an elevated post-treatment level is not a reliable indicator of persistence, proven by the fact that 46% of clinical responders fell in this category. Therefore, Cyfra 21-1 has a very limited role in correlating with response to treatment in carcinoma of the cervix.