The present paper is related to the frequently discussed question as to whether urethane tumorigenesis is a one stage or a multistage process. In either case, the tumorigenic process is assumed to begin with what may be called an initialevent, a change in a single normal cell (mutation) resulting from a single hit by a tumorigenic molecule (one hit theory) or from several such hits (multihit theory). If the initial event is followed by the growth of the tumor studied, then the mechanism is described as a one stage mechanism. However, as explicitly suggested by Brues , the growth (of first order mutants) following an initial event may well be benign in the sense of being destined to disappear, except for the possibility of a second mutation in one of its cells creating second order mutants. If this second mutation in a cell of the benign growth turns into a tumor cell, then the process of tumorigenesis is called a two stage mechanism. It is easy to visualize three or four or, generally, multistage mechanisms of tumorigenesis. Naturally, there is the possibility that, with respect to some particular tumors, say pulmonary adenomas in mice, the tumorigenic process is a one stage process while, with respect to some other tumors, say pulmonary carcinomas, it is a multistage mechanism.