Mean argyrophil nucleolar organizer regions (Ag-NORs) per nucleus were determined on columnar epithelium, bronchial dysplasia, roentgenographically occult squamous cell carcinoma and squamous cell carcinoma with roentgenographically findings in 76 formalin-fixed, paraffin-embedded resected specimens. 1) Mean Ag-NOR counts per nucleus (mean +/- standard deviation) were 1.67 +/- 0.30 in 10 columnar epitheliums, 2.74 +/- 0.41 in 13 lesions with bronchial dysplasia, 3.75 +/- 0.59 in 57 lesions with roentgenographically occult squamous cell carcinoma and 4.44 +/- 0.51 in 14 lesions with squamous cell carcinoma with roentgenographically findings. There were significant differences between each two groups. 2) In lesions with squamous call carcinoma, mean Ag-NOR counts per nucleus of T2, T3 or T4 tumor were significantly higher than those of T1 tumor. Mean Ag-NOR counts per nucleus of N1 or N2 tumor were significantly higher than those of N0 tumor.