Female BALB/c nu/+ mice, pregnant by nu/+ males (nu: gene for hairlessness-athymia) were given injections of urethan, a transplacental tumorigen, on Day 17 or 19 of gestation. After an average of 16 weeks under clean conventional conditions, the incidence of primary lung tumors was similar in nude and normal offspring treated with carcinogen on either gestational day, with a higher incidence after treatment of Day 19. Thus, the absence of thymus did not affect the occurrence of transplacentally induced primary lung tumors or alter the well-known perinatal increase in sensitivity. Histologically, the nu/nu tumors differed from normal in the appearance of many atypical basophilic cells and in a tendency to invade both the parenchyma and the pleural surface. These results suggested progression of the lung adenomas to a more atypical, invasive form, a progression that may have occurred prematurely in the absence of thymus-dependent immune response.