Oxygenation status and tumor vascularization seem to be important factors in determining therapeutic effectiveness and patient prognosis. An abundance of data on tumor oxygenation and vascularization is available and it clearly shows that most human solid tumors are heterogeneously oxygenated and vascularized. They contain hypoxic regions. Such regions and areas of reduced vascularization can affect the response to a variety of drugs. Direct measurements of pO(2) and the vascular density in various types of tumors have, upon correlation of the data to therapeutic outcome, shown that low pO(2) values and low vascular density are associated with a decreased response to therapy. Therefore, oxygenation status and the extent of tumor vascularization may well be important factors contributing to the difficulty of successful therapy in certain types of tumors.