The first case of primary Wilms' tumor of the uterus is described. It occurred in a 14-year-old girl. Microscopically, embryonal tubules, glomeruli, and blastema were identified within a myxomatous stroma. No teratomatous elements were found. The tumor arose in the uterus, permeated the endocervical canal, filled the vagina, and infiltrated the Douglas cul-de-sac and the left broad ligament through the posterior vaginal fornix. Comments are made on the histogenesis and histologic pattern of the tumor.