The chemotherapeutic approach to hormone-refractory metastatic prostate cancer (MHRPC) for a long time included only estramustine. Then, attempts have been made with other various agents as cyclophosphamide, vinblastine, etoposide, taxanes and carboplatinum. Although the new drugs and combinations have increased the response rate of MHRPC, they have had no impact on the natural history of MHRPC, which is about 1 year as median time of survival. After an occasional observation of prolonged response in a patient with MHRPC treated with a very well tolerated oral low-dose of cyclophosphamide, from February 1996 to October 2002, seven more patients with MHRPC and progressive disease were consecutively recruited. Response to treatment was evaluated by conventional radiological procedures and/or serial serum PSA measurements. The decline of PSA value was considered to assess the response consistent with the response guidelines from the prostate specific antigen-working group. All eight studied patients continuously received oral low dose cyclophosphamide until progression or the occurrence of significant toxicity. So far three patients (37.5%) progressed (PD), two (25%) showed PR and the three remaining SD. Response rate was 25%, and clinical benefit occurred in 62.5% of the studied patients. In the five patients with clinical benefit on cyclophosphamide median duration of clinical benefit, PR and SD were 9, 24+ and 8 months, respectively. In these five patients median overall survival times from cyclophosphamide and from the first regimen of chemotherapy were 17 and 33+ months respectively, while in the three patients with PD they were 4 and 13 months. The same interval times in patients with > or =50% decline of serum PSA were 29 and 50.5 months, while in those with <50% decline of the same marker, they were 13 and 32 months, respectively. Grade 2 or 3 neutropenia were observed in all the studied patients. In four (50%) of them pulmonary and urinary infections that were easily cured by the common antibiotics occurred. These data suggest that the metronomic use of cyclophosphamide, given alone, has similar or higher activity with lower toxicity than when administered with other active drugs. So it can be an useful option before or after the use of other single or combined potentially active chemotherapeutic agents.