Assessment of "squamous cell carcinoma antigen" (SCC) as a marker of epidermoid carcinoma of the anal canal. We measured squamous cell carcinoma antigen (SCC) in epidermoid carcinoma of the anal canal in 66 patients. Samples were taken at diagnosis, before treatment, and during follow-up; 353 samples were analyzed. The positive threshold was taken as 2 ng/ml. At diagnosis, the sensitivity of the marker was 44 percent and its specificity 92 percent. In our series, the pretherapeutic level of SCC does not correlate with T as in Papillons' Clinical Staging System, but it does correlate with nodal invasion (P less than 0.05). It is of no prognostic value at the time of diagnosis. During follow-up, at relapse the level of SCC is 20.3 +/- 43 ng/ml. This increase is significant (P less than 0.01): the sensitivity of the marker is 77 percent. In patients who have relapsed, development of the illness correlates with the level of SCC, which is of prognostic value (P less than 0.01). In conclusion, the level of SCC should be associated with the clinical follow-up of patients with epidermoid carcinoma of the anal canal. 