We evaluated the efficacy and safety of metronomic oral cyclophosphamide (CTX) and prednisone in metastatic castration-resistant prostate cancer (mCRPC) patients. We analyzed retrospectively patients with mCRPC previously treated with docetaxel, and who received metronomic CTX (from 50 mg PO daily to 150 mg PO, 14 days/7 days off) and prednisone 10 mg PO daily between September 2009 and April 2014 were analyzed. The primary endpoint was prostate-specific antigen (PSA) decrease ≥50 %. Secondary analysis included PSA decrease ≥30 %, time-to-treatment failure (TTF) and toxicity. Demographics and baseline characteristics were summarized using descriptive statistics. PSA response and adverse events were reported as relative rates. Kaplan-Meier estimates were calculated and plotted for time-to-event endpoints. Forty patients were evaluated. The median age was 69 years old (52-86), 12 (30.0 %) patients presented a Karnofsky performance status (KPS) of <80 %, and 34 (85 %) presented with bone with or without nodal metastases. Median pre-treatment PSA was 192 ng/dL (7-2696 ng/dL). All patients were previously exposed to docetaxel, including 33 (82.5 %) with docetaxel-refractory disease. PSA response rate was achieved in eight (20.0 %) out of 40 patients. Additionally, PSA declines of ≥30 % occurred in 14 (35.0 %) patients. The median TTF was 3 months (95 % confidence interval 2.5-3.5). The treatment was well tolerated. Grade 3/4 lymphopenia was reported in 11 (27.5 %) patients and was the only grade 3-4 toxicity reported. Metronomic oral CTX showed activity and safety in docetaxel-pretreated mCRPC patients. This regimen deserves further investigation in this setting.