Tumor-associated antigen has shown promise as a clinical aid in the detection and monitoring of uterine cervical squamous cell carcinoma. Antigen levels have been shown to reflect the extent of disease and response to treatment. These findings have suggested that measurements of tumor-associated antigen may be useful in monitoring other squamous cell carcinomas. To test this hypothesis, we measured tumor-associated antigen using the squamous cell carcinoma radioimmunoassay in 103 patients with previously treated squamous cell head and neck tumors and 28 patients with known squamous cell carcinoma of the head and neck. Increased squamous cell carcinoma antigen levels were found in 39 percent of patients with known tumors and in 19 percent of the patients with previous curative resection. The sensitivity of the assay limited its usefulness in predicting the presence of new and recurrent tumors.