In order to evaluate the alterations of nuclear p53 accumulation in early stage adenocarcinomas of the lung, nuclear p53 accumulation by small-sized peripheral adenocarcinomas of the lung was examined immunohistochemically. Peripheral adenocarcinomas of the lung, 2 cm or less in diameter, have been classified into two groups; one showing replacing growth of the pulmonary alveolar structure and the other showing non-replacing growth. The former group has been subdivided into three microscopic subtypes: type A, localized bronchioloalveolar carcinoma (LBAC); B, LBAC with foci of pulmonary alveolar structural collapse; and C, LBAC with foci of active fibroblastic proliferation. Type C is thought to be advanced carcinoma, which develops progressively from types A and B. Two of 32 (6%) types A and B carcinomas, 37 of 133 (28%) type C carcinomas and 14 of 35 (40%) non-replacement-type adenocarcinomas showed positive nuclear staining for p53. The positive staining frequency was significantly higher for type C than for types A and B (P < 0.05). These results suggest that nuclear p53 accumulation occurs in the transition from the early to advanced stages of replacement-type adenocarcinoma development and it may be a clinically useful indicator of the degree of tumor malignancy.