Some dysplasias in the bronchial epithelium are thought to be precancerous lesions that can develop into squamous cell carcinomas. In this investigation, we assessed the biological behavior of bronchial squamous dysplasia in order to define which dysplasias have the potential to progress to squamous cell carcinoma. Using autofluorescence bronchoscopy, we followed up periodically localized dysplasias and examined for correlation between histological outcome and smoking status during the follow-up period, telomerase activity, Ki-67 labeling index, and p53 immunoreactivity of initial biopsy specimens. Ninety-nine dysplasias from 50 participants mainly with sputum cytology suspicious or positive for malignancy were followed up. Of 99 dysplasias, 3 dysplasias progressed to squamous cell carcinoma, 41 dysplasias remained as dysplasia, 6 dysplasias changed to metaplasia, 14 dysplasias changed to hyperplasia, and 35 dysplasias regressed to bronchitis or normal bronchial epithelium. There were no significant associations between histological outcome and smoking status. Mean initial telomerase activity and Ki-67 labeling index values in the dysplasias increased in proportion to the severity of the histological outcome at the second biopsy. There was also a significant difference between p53-positive and p53-negative dysplasia in terms of histological outcome at the second biopsy. Our results suggested that dysplasias with high telomerase activity, increased Ki-67 labeling index, and p53-positivity tended to remain as dysplasia and might have the potential to progress to squamous cell carcinoma. Patients with dysplastic lesions with these characteristics should be carefully followed up.