5-Chlorouracil is a chlorinated derivative of the pyrimidine nucleoside base uracil .

In vivo, it is converted into chlorodeoxyuridine, which is mutagenic and genotoxic. Uracil is chlorinated at the 5 position in a cell-free myeloperoxidase, peroxide, and chloride system in which hypochlorous acid is formed. 5-Chlorouracil has been found in human neutrophils stimulated with phorbol 12-myristate 13-acetate in vitro and in inflammatory human exudate isolated from sites of superficial infection. Levels of 5-chlorouracil are increased in exudate isolated from the site of inflammation in a rat model of carrageenan-induced inflammation and in patient-derived human atherosclerotic aortic tissue.