Background: mCSPC is clinically complex. Although consensus on treatments are evolving, ADT remains the backbone of therapy. This study assessed the proportion of mCSPC patients treated with ADT only or remaining untreated and their HRU during the mCSPC period in the US. Methods: The Optum Clinformatics Extended DataMart was used to identify men with ≥2 claims for prostate cancer (PC), ≥1 claim for metastasis, ≥1 castration sensitivity (CS) indicator (CS diagnosis code [dx]; castration and no prostate-specific antigen [PSA] rise; or hormone/castration naive for ≥18 months [mo] before index [date of 1^st metastasis dx on or after 1^st PC dx and between 2015-2018]). Patients were excluded if they had a pre-index castration-resistance (CR) indicator (CR dx; castration within ≥90 days pre-index or with PSA rise; or a claim for a drug solely recommended for metastatic CRPC). mCSPC period (F/U) was defined as time from index until CR (i.e., any post-index CR indicator or initiation of abiraterone acetate or docetaxel ≥12 mo after post-index ADT initiation or ≥12 mo post-index for those with no ADT) or end of data. The proportion of patients receiving ADT only or no mCSPC treatment during F/U was reported. Per-patient-per-year (PPPY) all-cause HRU were evaluated during baseline (12 mo pre-index) and F/U. Descriptive statistics were used: n (%) for binary and mean [SD] for continuous variables. Results: A total of 2,825 mCSPC patients were identified (age: 75 [9] years). Of these, 2,181 (77%) received ADT only or no treatment in a F/U of 10.9 [9.0] mo. Among them, there were more patients with ≥1 inpatient (IP) stay or ≥1 emergency room (ER) visit (F/U vs. baseline; IP: 50% vs. 20%; ER: 57% vs. 44%), and patients had more IP stays and ER visits (IP: 2.0 [4.0] vs. 0.3 [0.7] stays; ER: 3.2 [7.1] vs. 1.1 [2.2] visits) and more IP days (27 [61] vs. 3 [11] days) PPPY in F/U vs. baseline. Trends were similar among patients receiving ADT only (N=1,252 [44%]; F/U of 12.6 [9.0] mo; Table). Conclusions: The majority of mCSPC patients were treated with ADT only or remained untreated and incurred substantial HRU. These findings suggest that improvements in therapy and prompt treatment initiation in men with mCSPC are needed to improve outcomes.