This compound can be obtained by three different ways: 1) The reaction of 1-(p-chlorobenzoyl)-2-methyl-5-methoxy-3-indolylacetic acid (indomethacin) (I) with SOCl2 gives the corresponding acetyl chloride (II), which is treated with hydroxylamine hydrochloride and NaOH in water to afford 1-(p-chlorobenzoyl)-2-methyl-5-methoxy-3-indolylacetohydroxamic acid (III). Finally, this compound is hydrolyzed with 0.5 N NaOH in water. 2) The reaction of 1-(p-chlorobenzoyl)-2-methyl-5-methoxy-3-indolylacetal-dehyde (IV) with N-hydroxybenzenesulfonamide (V) in DMSO by means of NaOH yields compound (III) already obtained. 3) By reaction of tert-butyl-5-methoxy-2-methyl-3-indolylacetate (VI) with hydroxylamine in methanol.