The swelling and shrinking behaviors of a series of poly(N-isopropylacrylamide) (PNIPA) hydrogels are studied in aqueous solutions of sodium dodecylbenzene-sulfonate (SDBS). Between 0 and 3 mol % 2-acrylamido-2-methylpropanesulfonic acid sodium salt (AMPS) is used as an ionic comonomer in the hydrogel synthesis. It is shown that the collapsed PNIPA gels in water at 52 degreesC start to swell above a critical SDBS concentration in the external solution. This critical concentration decreases as the ionic group content of PNIPA gel increases. A comparison of the swelling and shrinking experiments in SDBS solutions indicates strong hysteresis behavior of PNIPA gels. A more diluted solution is required to make a swollen gel start to reshrink than to cause gel swelling. The results show strong attractive forces between the isopropyl groups of the PNIPA network and the DB groups of SDBS molecules. (C) 2002 John Wiley & Sons, Inc. J Appl Polym Sci 83: 1228-1232, 2002.