Peripheral lung cancers frequently possess a fibrotic focus or scar in their center or beneath the pleura. We reexamined 58 cases of peripheral lung cancer of less than 3 cm in diameter (48 adenocarcinomas, two large cell carcinomas and eight squamous cell carcinomas) which were removed surgically between June 1962 and July 1973. Analyses of the cases revealed that in adenocarcinoma with increased collagenization or hyalinization in the fibrotic focus, the degree of pleural invasion and incidence of lymph node metastasis and blood vessel invasion were greater and thus the prognosis of the patient was poorer than in cases with no or slight collagenization. The results also indicate that the characteristics of the central fibrotic focus are probably more important than the size of the tumor for estimating the prognosis of patients with peripheral adenocarcinoma of less than 3 cm in diameter. The same might be said of peripheral large-cell carcinoma. However, the prognostic importance of the fibrotic focus was not confirmed in cases of peripheral squamous-cell carcinoma. Although the central or subpleural scar has long been the basis of the scar cancer concept, alternate explanations were considered, namely, that scar formation occurred along with tumor development, rather than before, in the case of adenocarcinomas.