BACKGROUND: Extramammary Paget's disease is an uncommon intraepidermal adenocarcinoma with poorly defined clinical implications. OBJECTIVE: The purpose of this research was to estimate the risk of second primary neoplasms in patients with extramammary Paget's disease. DESIGN: This was a retrospective analysis of the Surveillance, Epidemiology, and End Results Registry (1973-2014). SETTINGS: The study included population-based cancer registries from the United States. PATIENTS: Patients who were diagnosed with anogenital Paget's disease were included. MAIN OUTCOME MEASURES: Risk of second primary development was measured. RESULTS: We identified 108 patients with anal Paget's disease, 421 patients with male genital (scrotum or penis) Paget's, and 1677 patients with female genital (vagina or vulva) Paget's. Median follow-up time was 5.9 years. The risk of developing colorectal adenocarcinoma was 18.5% for patients with anal Paget's disease. Eighty percent of colorectal adenocarcinoma diagnoses were synchronous (within 2 mo) to anal Paget's diagnoses, whereas metachronous tumors occurred at a median time of 2.4 years. Of patients with anal Paget's disease, 8.3% developed an anal adenocarcinoma or nonsmall cell cancer. In male patients with genital Paget's, the risk of proximal genitourinary malignancy was 9.7%, scrotal or testicular adenocarcinoma was 0.4%, and penile or scrotal squamous carcinoma was 1.7%. In female patients with genital Paget's, the risk of proximal genitourinary malignancy was 3.0%, vaginal or vulvar adenocarcinoma was 1.4%, and vaginal or vulvar squamous neoplasm was 1.0%. Five-year overall survival was 59.7%, 73.5%, and 80.7% in patients with anal, male genital, and female genital Paget's (p < 0.001). LIMITATIONS: The registry did not record surveillance schedule, provider specialty, or nonprocedural therapies for extramammary Paget's disease. CONCLUSIONS: In the largest published cohort of patients with extramammary Paget's disease, patients with anal Paget's had a much higher risk of both proximal and local neoplasms as compared with patients with genital Paget's. Patients with anal Paget's also experienced worse survival as compared with those with purely genital Paget's. See Video Abstract at http://links.lww.com/DCR/B20.