For patients with docetaxel-resistant hormone-refractory prostate cancer (HRPC) no standard chemotherapeutic treatment exists. In this study, we evaluate the efficacy of cyclophosphamide (CP)-based metronomic chemotherapy in this patient population. Patients with metastatic HRPC with disease progression under docetaxel-based chemotherapy were eligible. The primary endpoint was prostate-specific antigen (PSA) response. Secondary endpoints were survival and toxicity. Low-dose CP (50 mg/d) and dexamethasone (1 mg/d) were administered orally in a metronomic manner. Treatment was continued until disease progression or intolerable side effects occurred. Seventeen patients were enrolled in this study. The median follow-up was 12 weeks (range: 4-60). Median age was 68 years (range: 42-85). Median PSA at study entry was 134 ng/ml (range: 46.0-6554). Nine patients had a PSA response (median 44.4%), four patients >or=50% and five patients <50%. Eight patients had a PSA progression. Overall survival was 24 months. Five patients reported a decrease in bone pain after 4 weeks' treatment. No grade 3 and 4 toxicities were noted. In this study, low-dose metronomically administered CP demonstrated efficacy as a second-line treatment in patients with docetaxel-resistant HRPC. The treatment was well tolerated and almost without toxicity. Further advantages of low-dose CP were its convenient oral administration, dosing schedule, low cost, and low-toxicity profile. These attributes in combination with immunoregulatory and antiangiogenic potentials make CP also a prime candidate for combination with other treatment regimens.