Exposure to static magnetic fields (SMFs) results in a reduced blood flow in tumor vessels as well as in activation and adherence of platelets. Whether this phenomenon may have a significant functional impact on tumors has not been investigated as yet. The aim of our study was to evaluate the effects of prolonged exposure to SMFs on tumor angiogenesis and growth. Experiments were performed in dorsal skinfold chamber preparations of Syrian Golden hamsters bearing syngenic A-Mel-3 melanomas. On 3 d following tumor cell implantation one group of animals was immobilized and exposed to a SMF of 586 mT for three h. Control animals were immobilized for the same duration without SMF exposure. Using in vivo-fluorescence microscopy the field effects on tumor angiogenesis and microcirculation were analyzed for seven days. Tumor growth was assessed by repeated planimetry of the tumor area during the observation period. Exposure to SMFs resulted in a significant retardation of tumor growth ( approximately 30%). Furthermore, histological analysis showed an increased peri- and intratumoral edema in tumors exposed to SMFs. Analysis of microcirculatory parameters revealed a significant reduction of functional vessel density, vessel diameters and red blood cell velocity in tumors after exposure to SMFs compared to control tumors. These changes reflect retarded vessel maturation by antiangiogenesis. The increased edema after SMF exposure indicates an increased tumor microvessel leakiness possibly enhancing drug-uptake. Hence, SMF therapy appears as a promising new anticancer strategy-as an inhibitor of tumor growth and angiogenesis and as a potential sensitizer to chemotherapy.