Background and objective: The depth of the prostate-specific antigen (PSA) decline after androgen receptor pathway inhibitor (ARPI) treatment combined with androgen deprivation therapy for patients with metastatic hormone-sensitive prostate cancer (mHSPC) may affect prognosis. The primary objective in our study was the correlation between the PSA response at 3 mo and radiologic progression-free survival (rPFS) at 24 mo. Three groups were defined according to the PSA decline: complete response (PSA <= 0.02 ng/ml), partial response (PSA >0.02 and <= 0.2 ng/ml), and incomplete response (PSA >0.2 ng/ml). Secondary objectives were correlation between the PSA response at 3 mo and overall survival, and the development of a model predicting complete PSA response. Methods: We conducted a retrospective multicenter study of patients with mHSPC treated with apalutamide from May 2018 to September 2023 registered in the Real-World Evidence APA registry across 20 centers. Key findings and limitations: We included 633 patients with mHSPC. The median age at diagnosis was 68 yr (interquartile range [IQR] 63-75) and median PSA was 16 ng/ml (IQR 7.5-64). Some 63% of the short had low-volume disease, 51% had de novo disease, 48% had recurrent disease. At 3 mo, 27% had a complete response, 42% a partial response, and 31% an incomplete response, with corresponding rRFS rates at 24 mo of 92%, 86%, and 63%. According to the predictive model, a complete PSA response at 3 mo was associated with the use of next-generation imaging and PSA <50 ng/ml at diagnosis. Study limitations include heterogeneity among the groups and variations in data quality and assessment methods. Conclusions and clinical implications: Patients with a complete PSA response after 3 mo of apalutamide treatment face a very low risk of progression within 2 yr. Conversely, nearly 50% of patients with an incomplete PSA response will experience disease progression. Patient summary: For patients with metastatic prostate cancer that is still responsive to hormone therapy, a complete response after treatment with a drug called apalutamide is associated with a very low risk of progression within 2 years. However, nearly half of patients with an incomplete response to apalutamide will experience progression of their cancer. (c) 2024 The Author(s). Published by Elsevier B.V. on behalf of European Association of Urology. This is an open access article under the CC BY-NC-ND license (http://creative- commons.org/licenses/by-nc-nd/4.0/).