Background: Ketoconazole for the treatment of metastatic castration resistant prostate cancer (mCRPC) has never been investigated in randomized trials. In fact, although ketoconazole is a less potent inhibitor of the CYP-17 enzyme systems compared to abiraterone, there is no head-to-head evidence demonstrating that this translates into improved survival. We analyzed biochemical response rates in post-chemotherapy mCRPC patients who were treated with ketoconazole 200 mg TID plus prednisone 5 mg BID at a single center in Denmark. Methods: Between 2008 and 2012, where neither abiraterone nor enzalutamide was available in Denmark, a total of 30 post-chemotherapy mCRPC patients were managed with ketoconazole. Percent change in prostate-specific antigen (PSA) and alkaline phosphatase (ALP) from baseline were registered. Overall survival was calculated from the initiation of ketoconazole. Results: At initiation of ketoconazole, median age was 66 (range: 58 to 76). Median PSA was 547 ng/ml (65 to 4,241). Median ALP was 159 U/L (51 to 750). The median number of prior cancer therapies was five (three to nine). The median time on ketoconazole treatment was 150 days (14 to 648). A total of 44% patients had any decline in PSA, whereof 31% experienced a decline of >=50%. At three months, a total of 19 patients still received ketoconazole, whereof 26% sustained a PSA decline of >=50% from baseline. A total of 71% experienced any decline in ALP, whereof 36% experienced a maximum decline of >=50%. At three months, 7 of 19 patients (36%) sustained an ALP decline of >=50% from baseline. At follow-up, 28 of 30 patients had died. The median overall survival was 10.5 months (95%CI: 8.3-12.6). Conclusions: Compared to the post-chemotherapy phase II study of abiraterone (58 patients) our cohort most likely had a larger tumor burden (e.g. higher PSA, more therapies prior to start of ketoconazole). Nonetheless, we demonstrated a reasonably comparable maximum >=50% PSA response-rate (43% in phase II) and three-months response-rate >=50% (36% in phase II). Also a significant decline in ALP was demonstrated here which has not been reported in abiraterone trials. Given the large difference in expenditure, a head-to-head comparison of abiraterone and ketoconazole in mCRPC patients still seem justified.