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BACKGROUND The aim of this study was to evaluate the association between periprostatic fat thickness (PPFT) and time to castration-resistant prostate cancer (CRPC) in newly diagnosed patients with prostate cancer (PCa) treated with androgen deprivation therapy (ADT). PATIENTS AND METHODS We retrospectively reviewed the medical records of 150 patients with PCa treated with ADT at our hospital between June 2011 and June 2017. PPFT measured on magnetic resonance imaging (MRI) and PPFT/periprostatic fat volume (PPFV) measured on computed tomography (CT) were evaluated. Kaplan-Meier curves and log-rank tests were used to assess significant differences in time to CRPC between the 2 groups (high PPFT vs. low PPFT, determined by PPFT > or < the median value, respectively). Univariable and multivariable Cox regression analyses were employed to identify the potential prognostic factors for survival. RESULTS The median value of PPFT measured on MRI was 0.555 cm. PPFT was significantly associated with PPFV measured on CT images (with a correlation coefficient of 0.825; P < .001). A total of 66 patients (44%) progressed to CRPC during the follow-up period. Patients with high PPFT (measured on MRI) showed a significantly shorter PFS than patients with low PPFT. Multivariable Cox analysis demonstrated that T stage, presence of distant metastasis, shorter time to prostate-specific antigen nadir, higher prostate-specific antigen nadir, Gleason score (greater than 4 + 4), and high PPFT were significantly associated with shorter PFS. CONCLUSIONS PPFT is significantly associated with PPFV measured on CT images. PPFT measured on MRI is a readily available and significant predictor of time to CRPC in patients with PCa receiving ADT as the primary treatment.