SignificanceThe androgen receptor (AR) antagonist enzalutamide is one of the principal treatments for men with metastatic castration-resistant prostate cancer. However, not all patients respond, and de novo resistance mechanisms are largely unknown. To clarify mechanisms that contribute to enzalutamide resistance, we conducted a single-arm enzalutamide clinical trial. Metastatic tissue biopsies were required prior to study entry so we could attempt to identify molecular features associated with de novo resistance. Transcriptional profiling identified specific gene sets—including those linked to reduced AR transcriptional activity and a stemness program—that were activated in nonresponders. Our results suggest that patients whose tumors harbor this program should be considered for clinical trials testing rational agents to overcome de novo enzalutamide resistance.