Background: The somatic mutation HSD3B1(1245A > C) has been mechanistically linked to castration-resistant prostate cancer by encoding a mutant enzyme that augments intratumoral dihydrotestosterone (DHT) synthesis. Given the HSD3B1(1245C) allele is also frequently found in the germline, we hypothesized men inheriting this variant allele would exhibit resistance to androgen deprivation therapy (ADT), as manifested by worse clinical outcomes. Methods: We used a large, prospectively maintained prostate cancer registry to identify men treated with ADT for biochemical failure in the post-prostatectomy setting who were without evidence of metastatic disease. We analyzed progression-free survival (PFS), distant metastasis-free survival (DMFS), and overall survival (OS) according to HSD3B1 genotype using Kaplan-Meier methods. Cox proportional hazards regression was performed to evaluate potential gene-dosage effects, with homozygous wild-type men serving as the reference group. Demographic and treatment characteristics were compared to assess for possible confounders using Fisher’s exact test and Kruskal-Wallis analysis of variance. Multivariable analysis (MVA) was performed to assess whether HSD3B1 genotype independently predicted clinical outcomes. Results: Of 118 men genotyped, 37% were homozygous wild-type, 53% were heterozygous, and 10% were homozygous variant. Demographic and treatment characteristics did not differ across groups. Median PFS diminished as a function of the number of variant alleles inherited (6.6 years in homozygous wild-type men, 4.1 years in heterozygotes, and 2.5 years in homozygous variant men; P = 0.01). Median DMFS likewise decreased according to the number of variant alleles inherited (9.1 years vs. 6.8 years vs. 3.6 years, respectively; P = 0.01). Finally, OS similarly diminished (5-year and 10-year OS: 82% and 55% vs. 74% and 35% vs. 58% and 0%, respectively; P = 0.006). On MVA, the associations between HSD3B1 genotype and metastasis (hazard ratio (HR) 2.76; P = 0.023) and death (HR 3.33; P = 0.016) were maintained. Conclusions: Inheritance of the variant HSD3B1(1245C) allele that enhances DHT synthesis may be a powerful predictor of resistance to ADT for prostate cancer.