Squamous cell carcinoma (SCC) of the bronchus is considered to develop from preneoplastic 'dysplasia', but reports of sequential observation of this dysplasia-carcinoma sequence in humans are very few. We followed four dysplastic lesions found in the bronchi of three ex-chromate workers by bronchoscopy and biopsy and found that all of them progressed to SCC. Of the four lesions, three were severe dysplasias at the first biopsy which progressed to SCCs in 7-13 months. The last one was a slight dysplasia at the first biopsy and showed progression of the atypia to carcinoma in 6 years and 10 months. An immunohistochemical analysis of the chronological change in p53 protein expression in these lesions and in normal ciliated epithelium taken from the surroundings was conducted in each case. Overexpression of p53 protein was observed in two of the severe dysplasias and the one slight dysplasia, as well as their eventual SCCs. However, no such change was apparent in one case of severe dysplasia or its eventual SCC. Normal epithelium was consistently negative. Our results provide direct proof of the dysplasia-carcinoma sequence and suggest that alteration in the expression of p53 protein might be an important early event which persists. Therefore, the immunohistochemical detection of p53 overexpression in biopsy specimens of bronchial epithelium might be useful for evaluation of preneoplastic lesions in high-risk group individuals and for early diagnosis of bronchial cancer.