The synthesis of the title compound, 3-hydroxyisoxazole-5-hydroxamic acid (4b), by 2 procedures is described. The 1st, involving the treatment of dimethyl acetylenedicarboxylate with hydroxylamine, was previously reported to give the 3-hydroxyisoxazole-5-carboxylic acid (4a). In the 2nd, treatment of chlorofumaroyl dichloride with hydroxylamine also gave the intermediate chlorofumarodihydroxamic acid (6). Compound 6 had some activity against P388 lymphocytic mouse leukemia.