The design, synthesis, and in vitro biochemical evaluation of a class of mechanism-based inhibitors of human leukocyte elastase (HLE) that incorporate in their structure a 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold with appropriate recognition and reactivity elements appended to it is described. The synthesized compounds were found to be efficient, time-dependent inhibitors of HLE. The interaction of the inhibitors with HLE is postulated to lead to the formation of a highly reactive N-sulfonyl imine (a Michael acceptor) that arises from an enzyme-induced sulfonamide fragmentation cascade. Subsequent reaction ultimately leads to the formation of a relatively stable acyl enzyme. The results cited herein demonstrate convincingly the superiority of the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold over other scaffolds (e.g., saccharin) in the design of inhibitors of (chymo)trypsin-like serine proteases.