Nucleolar organizer regions in precancerous and cancerous lesions of the bronchus. Using a silver staining technique, nucleolar organizer region-associated proteins (Ag-NOR) were studied in paraffin sections of five specimens of normal bronchial epithelium, eight of atypical squamous metaplasia, five of carcinoma in situ, and seven of microinvasive squamous cell carcinoma. The mean number of Ag-NOR in the nucleus were normal epithelium 1.2 +/- 0.1 (mean +/- SD), atypical squamous metaplasia (borderline lesion) 2.2 +/- 0.5, carcinoma in situ 3.8 +/- 0.6, and microinvasive squamous cell carcinoma 4.8 +/- 1.1. There was a highly significant difference between the Ag-NOR numbers in the atypical squamous metaplasia and those in the carcinoma in situ (P less than 0.01). The Ag-NOR staining is a useful technique for the differential diagnosis of difficult borderline lesions in the bronchial epithelium. 