Serum testosterone level is a useful biomarker for determining the optimal treatment for castration-resistant prostate cancer.

•We have provided evidence of the usefulness of serum testosterone as a biomarker for predicting castration-resistant prostate cancer (CRPC) treatment outcomes.•Both abiraterone and enzalutamide exerted effects in patients who had CRPC progression and serum testosterone levels of 5 to 50 ng/dl,•However, abiraterone, but not enzalutamide, exerted effects even in those who had serum testosterone levels of <5 ng/dl.•The serum testosterone level should be considered to more precisely guide treatment selection for patients with CRPC.