Novel androgen receptor signaling inhibitors for prostate cancer (PC) impose the burden of self-administration on older patients overwhelmed by the requirement of many other concomitant medications. This study evaluated the proportion of non-adherence in a 12-month follow-up period and the first 3 months to abiraterone (ABI) or enzalutamide (ENZ). In a prospective multicenter observational cohort study, patients with metastatic castration-resistant PC (mCRPC) aged ≥70 years receiving ABI or ENZ pre- or post-docetaxel were enrolled. Treatment monitoring included pill counting, a self-assessment questionnaire, and clinical diaries at each clinical visit. Non-adherence rates were based on proportions of missed/prescribed pills ratios by pill counting. Overall, 234 patients were recruited with median age of 78 years (range, 73-82); 86 (37%) were treated with ABI, and 148 (63%) with ENZ. The median follow-up for adherence was seven monthly cycles (IQR: 4-12). The two cohorts were well balanced for baseline characteristics. The percentage of non-adherence by pill counting was slightly higher for ABI than ENZ (5.2% vs. 4.2%, P < .001). By self-reporting, patients on ENZ tended to report more frequently than those with ABI forgetfulness as the reason for missing events (42% vs. 17%, P < .001). A lower Geriatric G8 score correlated with non-adherence (P = .004). Overall survival (OS) was 48.8 months. Patients on ABI had radiographic progression-free survival (rPFS) of 28.4 24.2-32.5, while for ENZ patients, we reported a median rPFS of 23.1 18.2-28.1 months. Physicians tend to treat older mCRPC patients with ENZ. Non-adherence rate is relatively low overall but can be higher with ABI than with ENZ and correlates with the Geriatric G8 score. Forgetfulness is a potential barrier for ENZ.