['To explore the prognostic and predictive value of baseline variables in 512 patients with metastatic castration-resistant prostate cancer from the phase III Immunotherapy for Prostate Adenocarcinoma Treatment (IMPACT) trial who were randomized to receive sipuleucel-T or control.', 'The most powerful of these prognostic factors, baseline prostate-specific antigen (PSA), was subdivided into quartiles to evaluate treatment effect patterns. Cox regression analyses were used to assess predictors of overall survival (OS) and sipuleucel-T treatment effect within PSA quartiles. Median OS was estimated by the Kaplan-Meier method.', 'PSA was the strongest baseline prognostic factor (P\xa0<.0001). Furthermore, the sipuleucel-T treatment effect appeared greater with decreasing baseline PSA. The OS hazard ratio for patients in the lowest baseline PSA quartile (≤22.1 ng/mL) was 0.51 (95% confidence interval, 0.31-0.85) compared with 0.84 (95% confidence interval, 0.55-1.29) for patients in the highest PSA quartile (>134 ng/mL). Estimated improvement in median survival varied from 13.0 months in the lowest baseline PSA quartile to 2.8 months in the highest quartile. Estimated 3-year survival in the lowest PSA quartile was 62.6% for sipuleucel-T patients and 41.6% for control patients, representing a 50% relative increase.'] Is lower baseline prostate-specific antigen associated with a greater overall survival benefit from sipuleucel-T in the Immunotherapy for Prostate Adenocarcinoma Treatment ( IMPACT ) trial?