Comparison of reported series of laryngeal lesions is complicated by the inconsistency in the terminology. The classification of these lesions should logically be based on the degrees of dysplasia, as this has a bearing on the prognosis. The material for this study consisted of 193 patients with hyperplasia and/or keratosis, with or without mild dysplasia (Group I), moderate dysplasia (Group II), and severe dysplasia and carcinoma in situ (Group III). They were treated over a 14-year period (1966-1979) at Linköping University Hospital. Of the 98 patients in Group I available for follow-up, 23 had repeat excisions. Moderate dysplasia developed in 3 patients, severe dysplasia in 2, and invasive carcinoma in another 2 within 3 and 3.5 years; they both underwent laryngectomy. Among the 24 patients in Group II available for follow-up, severe dysplasia developed in 3 and invasive carcinoma in 3 up to 13 years after the initial diagnosis, all but one (with severe dysplasia) received a full course of radiotherapy, on one case total laryngectomy was subsequently performed for recurrent carcinoma. Of the 39 patients with severe dysplasia and carcinoma in situ 16 were given primary radiotherapy; 4 of these developed invasive carcinoma; 3 of them underwent total laryngectomy and one partial laryngectomy. Excision or stripping of the vocal cords was the primary treatment in 23 cases; 5 of these developed invasive carcinoma; 2 of them underwent total laryngectomy. Diffuse lesions and well differentiated forms of severe dysplasia caused most problems and had the highest incidence of invasive carcinoma irrespective of the form of treatment. Patients with hyperplasia, keratosis, dysplasia and carcinoma in situ should be carefully followed up, as invasive carcinoma may develop many years after the initial diagnosis.