Prostate cancer (PCa) is a major cause of disease and mortality among men worldwide. In 2020, approximately 1.4 million men were diagnosed with PCa, and approximately 375,000 men died of PCa. We urgently need biomarkers for diagnosis, prognosis, therapy prediction, or therapy monitoring. Liquid biopsies, including cell-free DNA (cfDNA) and circulating tumor cells (CTCs), are valuable for studying such biomarkers and are minimally invasive. We investigated plasma cfDNA from 34 progressive PCa patients for sequence variants. We found that analysis of cfDNA and CTCs can provide complementary information that may in future support treatment decisions and choice of therapy options for advanced PCa patients. We also investigated protein expression of AR and AR-V7 in PCa patients. Immunohistochemical staining of specimens from 410 patients showed that AR-V7 protein in granular cytoplasmic structures is an independent prognostic factor for relapse-free survival.