We analyzed the p53 protein immunohistochemically in bronchial dysplasias or squamous cell carcinomas in situ and in squamous cell lung carcinomas occurring in the same patients. The polyclonal antibody used (CM-1) is directed against the wild-type p53 protein, but also recognizes the mutated p53 in formalin-fixed and paraffin-embedded sections. To study the integrity of basement membranes (BMs) and the possible invasion of the dysplastic epithelium, immunostainings for the BM proteins laminin and type IV collagen were used. Nine of the 17 dysplasias showed p53 protein expression (53%); it was significantly more often seen in severe dysplasias and carcinomas in situ than in mild or moderate dysplasias (P = 0.04). The p53 antigenicity was generally located in the basal part of the epithelium. The BMs beneath mildly dysplastic epithelia were continuous. In contrast, those under moderately or severely dysplastic epithelia showed occasional disruptions. p53 protein expression was also found in dysplastic epithelium above a continuous BM suggesting an ominous process before signs of invasion. Twelve of the 17 squamous cell carcinomas showed p53 protein expression (71%). There was a significant concurrent p53 expression in bronchial dysplasias and their related squamous cell carcinomas (P = 0.009), so that all nine cases of p53 positive bronchial dysplasia also showed p53 positivity in the associated squamous cell carcinomas. These findings indicate that p53 protein expression is possible in premalignant bronchial lesions, and suggests that the p53 expression could, at least in some cases, be an early event in the development of a squamous cell carcinoma of the lung.