A method for radiosensitizing hypoxic tumor cells in a warm-blooded animal is provided. The method involves administration of a nicotinamide compound having the structural formula ##STR1## wherein X is O or S, and Z is H, OR, SR or NHR where R is as defined herein. Exemplary compounds for use in conjunction with the disclosed method include: 5-hydroxynicotinamide; 5-hydroxynicotinamide, sodium salt; 5-(2,3-dihydroxypropoxy)nicotinamide; 5-aminonicotinamide; 5-(2-methoxyethylamino)nicotinamide; and 5-acetamidonicotinamide.