Hypoxic cells in tumors are proposed to consist of at least 2 types, depending on whether they remain hypoxic for long (chronic hypoxia) or short (acute hypoxia) periods. Experimental evidence of the possible presence ofacutely-hypoxic cells in one type of murine tumour is presented. Finally, the possible implications for radiotherapy and chemotherapy of the presence of acutely-hypoxic cells in human tumors is discussed briefly.