Clinicopathological features of surgically-treated peripheral T1 adenocarcinoma of the lung were compared between 30 females and 26 males, and the following sex differences were observed; 1) Females were younger than males, 2) There was a higher percentage of smokers among males, 3) The acinar histologic subtype was less frequently found in females, 4) Well differentiated tumors were more frequently found in females, 5) K-ras gene mutations were observed only in males, 6) Prognosis was slightly better in females. As to other factors such as N-factor, tumor ploidy or central fibrosis, there was no statistical difference between the two groups. Although we were not able to explain the causes for the above sex differences, it was speculated that smoking was mainly responsible for them.