Dinitrochlorobenzene (DNCB) is a simple chemical useful to evaluate both afferent (sensitizing) and efferent (reacting) limbs of the delayed, cell-mediated immune system. This system invokes the small lymphocyte (T cell) and is apparently part of the host defense against cancer. Seventy-two patients with new primary squamous cell carcinomas of the larynx, hypopharynx and oropharynx were sensitized to DNCB by contact application, and delayed cutaneous hypersensitivity tested. The T cells were also assessed in this study by lymphocyte count. All patients were treated for cure. Ninety-one percent of DNCB reactors were alive and tumor-free at two years. Forty-five percent of negative reactors had recurrent tumor and died of carcinoma. Cases grouped by site among glottic, supraglottic and inferior hypopharynx showed identical results. Lymphocyte counts showed a trend in the same direction. On statistical analysis, all differences were significant. Eighty-five percent of recurrent tumors occurred in patients who were nonresponders. The present data suggest DNCB reactivity is helpful in predicting outcome after surgery, or combined radiation and surgery, for mucosal squamous carcinoma of the head and neck.