Background: Clinically localized prostate cancer can be treated with prostatectomy, radiation or focal therapy, with known impacts on sexual function. We sought to characterize long term sexual function in a large cohort of men treated with primary EBRT and to differentiate between those who have high vs low sexual function scores at baseline. Methods: Patients enrolled in CaPSURE, a national registry of men diagnosed with prostate cancer, from 1995 - 2017 with clinically localized prostate cancer (cN0M0/X, PSA < = 50) and treated with primary EBRT were included. Sexual function (SF) was self-reported using the UCLA Prostate Cancer Index (PCI). We performed repeated measures mixed models to evaluate SF after EBRT, stratified by baseline SF and adjusted for age, BMI, comorbidities, smoking, clinical risk, receipt of brachytherapy and/or hormonal therapy, and type of clinical site. Results: 1193 patients were included for analysis. Mean age was 70, mean number of comorbidities was 2, and median PSA was 7.5 at diagnosis. Baseline PCI scores were available for 43% of the cohort (data not collected before 1998). The mean PCI score was 38 out of 100 at baseline and after EBRT, declined gradually to 16 by 10 years. When only those men with good SF at baseline were assessed (scores 80 – 100), the initial rate of decline was more severe, but the nadir in this group was higher than those with lower baseline scores. Unadjusted mean PCI scores showed a similar decline amongst both groups, although those with good baseline SF consistently had better scores. 63% of patients were treated for erectile dysfunction (PDE5 inhibitors, intracavernosal injections or both), 13% of whom started therapy before EBRT. In the repeated measures mixed models analysis, we found that SF changed significantly over time and scores differed significantly by age, comorbidities, baseline SF, receipt of brachytherapy boost and concurrent ADT. Conclusions: SF after EBRT declines and persists. The most affected are those who had better PCI scores at baseline, although their overall PCI scores are consistently higher compared to those with baseline scores less than 80. Among both groups however, there was a similar decline that was statistically significant.