This study compared long-term urinary, bowel, and sexual function after radical prostatectomy or external-beam radiation therapy for localized prostate cancer. The Prostate Cancer Outcomes Study (PCOS) enrolled 3533 men diagnosed with prostate cancer in 1994 or 1995. Of these, 1655 men aged 55–74 years with localized prostate cancer underwent either surgery (n=1164) or radiotherapy (n=491). Functional status was assessed at baseline and at 2, 5, and 15 years after diagnosis. At 2 and 5 years, men who underwent prostatectomy were more likely to experience urinary incontinence and erectile dysfunction compared to those who received radiotherapy. However, at 15 years, no significant differences were observed in the adjusted odds of these outcomes between the two groups. Men undergoing prostatectomy were less likely to experience bowel urgency at 2 and 5 years compared to those undergoing radiotherapy, but no significant differences were noted at 15 years. The study found that men treated for localized prostate cancer commonly experienced declines in all functional domains over 15 years of follow-up. Despite these declines, no significant relative differences in disease-specific functional outcomes were observed at 15 years between men undergoing prostatectomy or radiotherapy. The results suggest that both treatments lead to similar long-term functional outcomes, but patients may experience functional declines over time regardless of treatment. The study highlights the importance of considering long-term functional outcomes when making treatment decisions for localized prostate cancer.This study compared long-term urinary, bowel, and sexual function after radical prostatectomy or external-beam radiation therapy for localized prostate cancer. The Prostate Cancer Outcomes Study (PCOS) enrolled 3533 men diagnosed with prostate cancer in 1994 or 1995. Of these, 1655 men aged 55–74 years with localized prostate cancer underwent either surgery (n=1164) or radiotherapy (n=491). Functional status was assessed at baseline and at 2, 5, and 15 years after diagnosis. At 2 and 5 years, men who underwent prostatectomy were more likely to experience urinary incontinence and erectile dysfunction compared to those who received radiotherapy. However, at 15 years, no significant differences were observed in the adjusted odds of these outcomes between the two groups. Men undergoing prostatectomy were less likely to experience bowel urgency at 2 and 5 years compared to those undergoing radiotherapy, but no significant differences were noted at 15 years. The study found that men treated for localized prostate cancer commonly experienced declines in all functional domains over 15 years of follow-up. Despite these declines, no significant relative differences in disease-specific functional outcomes were observed at 15 years between men undergoing prostatectomy or radiotherapy. The results suggest that both treatments lead to similar long-term functional outcomes, but patients may experience functional declines over time regardless of treatment. The study highlights the importance of considering long-term functional outcomes when making treatment decisions for localized prostate cancer.