Prostate cancer is the most commonly diagnosed cancer in American men. Adenocarcinoma accounts for over 95% of all prostate cancer cases. Prostate cancer is typically staged according to the American Joint Committee on Cancer's tumor, node, metastasis (TNM) system, in which the tumor stage (T) is based on the extent of penetration or invasion beyond the prostatic capsule into adjacent structures. Localized prostate cancer is classified as stages T1 (non-palpable) and T2 (palpable) and is confined within the prostatic capsule. The likelihood of progression to invasive cancer is associated with the presence of more poorly differentiated cells and other histopathologic features. This review focuses on the benefits and harms of treatments for screen-detected prostate cancer. However, many studies do not report how prostate cancer was initially detected. Therefore, we also included studies of treatments for localized (stages T1 and T2) prostate cancer, which is far more frequently detected by screening than more advanced cancer. Among newly diagnosed patients in 2004-2005, 94% had clinically localized prostate cancer. Screening with prostate-specific antigen (PSA) testing can detect prostate cancer in earlier, asymptomatic stages, when treatments might be more effective. In 2008, based on an earlier systematic review that focused on studies of PSA-based screening versus no screening, the U.S. Preventive Services Task Force (USPSTF) found insufficient evidence to determine the balance of benefits and harms associated with prostate cancer screening in men younger than age 75 years (I statement), and recommended against screening in men ages 75 years or older (grade D recommendation). Once prostate cancer has been detected by screening, treatments are frequently initiated. Understanding the benefits and harms associated with such treatments is therefore critical for informing screening decisions. Evidence on benefits and harms of treatments for localized prostate cancer was last reviewed by the USPSTF in 2002. This report summarizes the evidence on benefits and harms of treatment for screen-detected or early-stage prostate cancer, with an emphasis on studies published since 2002.