UV blood-irradiation (UBI) using the ImmunoModulator (IM) is in clinical studies for treatment of patients infected with human immunodeficiency virus (HIV) and hepatitis C virus (HCV). Preclinical studies suggest UBI could be also effective against cancer. To further explore this potential, a patient with an aggressive prostate cancer that has recently become androgen-refractory was treated with UBI using the IM. Cancer progression was monitored by means of prostate specific antigen (PSA). Prior to treatment with UBI PSA level increased exponentially over 6 months with a doubling time of 2.3 months. After treatment PSA level increased at a slower rate of 4.0 months doubling time. It is concluded that UBI treatment using the IM has an inhibitory effect on the growth of prostate cancer, presumably by stimulating a cellular immune response. Since androgen-refractory prostate cancer is invariably fatal, clinical studies with the IM are warranted for this indication.