At the Academic Hospital of the Free University of Brussels 243 patients underwent microlaryngoscopy over a period of 11 years (1978-1989). Histologic examination disclosed premalignant lesions in 55 patients. 26 patients had a follow-up (mean follow-up time was 5 years) and were selected for this study. The patients with premalignant lesions were divided into 3 groups: group 1 = hyperplasia and/or keratosis with or without mild dysplasia (11 patients); group 2 = mild dysplasia (7 patients); group 3 = severe dysplasia and carcinoma in situ (8 patients). In group 1 no malignant transformation was seen. In group 2 one invasive carcinoma developed after a period of 5 years (14%) and in group 3 two malignant transformations appeared 14 and 15 months after the initial diagnosis (25%). The treatment of choice was a total excision biopsy or stripping. A life time follow-up with an interval period of 6 months is recommended. The patients were also motivated to change their smoking habits.