A novel series of antiinflammatory agents, N-isoxazolyl-3-carboxamides of 4-hydroxy-2H-1,2-benzothiazine 1,1-dioxide, was synthesized and evaluated as antiinflammatory agents in the carrageenin-induced rat paw edema (CIRPE) assay and adjuvant-induced polyarthritis (AIP) assay. Several analogues were found to be equipotent or more potent than aspirin and phenylbutazone. Structure-activity relationships are discussed. One of the compounds, 4-hydroxy-2-methyl-N-(5-methyl-3-isoxazolyl)-2H-1,2-benzothiazine 3-carboxamide 1,1-dioxide (3a; isoxicam), was found to be 3 times as potent as phenylbutazone in the CIRPE and in the therapeutic AIP assays. Isoxicam (3a) is presently undergoing phase III clinical trial as an antiarthritic drug.