Cells staining positive for MIB (Ki-67) were observed in 1.8, 3.7, 1.3, 1.9, 1.7, and 2.7% of all alveolar epithelial cells in collapsed and mural fibrosis areas in UIP cases for which the SP-A positive ratios were 75.3, 64.3, 48.4, 7.7, 7.7, and 7.7%, respectively. The gene discussed is MKI67; the disease is idiopathic pulmonary fibrosis.