Some tumors in the larynx and pharynx have borders which can clearly be defined with the eye alone. In other patients the cancer is part of a field cancerization in which the main tumor may be accompanied by areas of microinvasive carcinoma, carcinoma in situ, or dysplasia, changes which may not be grossly recognizable. In an attempt to facilitate identification of such areas, a group of 48 laryngeal and pharyngeal cancer patients were studied. Topical in vivo staining with toluidine blue was utilized to assess the staining properties of the lesions. It was found that invasive carcinoma, carcinoma in situ, and areas of dysplasia showed a positive stain response. This procedure can be used to determine the extent of neoplastic change in pharyngeal and laryngeal mucosa.