Eight cases of Paget's disease of genital mucosa with malignancy of the lower urinary tract are described. In five it was apparent that there was concurrence of two separate malignancies. In one patient with long-standing Paget's disease of the genital mucosa, carcinoma of the urethra and bladder developed subsequently in continuity with the genital lesion; the two lesions were indistinguishable histologically, suggesting extension of the Paget's disease into urothelium. In two patients with bladder malignancy, there was histologic evidence of outward pagetoid extension of this process along urothelium and onto the genital mucosa. The significance of genital Paget's disease is discussed in the light of these findings, and the possible origins of Paget cells within the epidermis are reviewed.