The swelling behavior of poly(N-isopropylacrylamide) (PNIPA) gels in polymer solutions, particularly in aqueous solutions of poly(ethylene glycol)s, was investigated using the theory of equilibrium swelling. The volume of the PNIPA gel and the partition parameter of the macromolecules between the gel and the solution phases were calculated for various extents of the energetic interactions between the components. The simulation results were compared with the experimental data reported in the literature. It was shown that the PNIPA gel has a tendency to a reentrant phase transition in an aqueous solution of low molecular weight linear polymers. In such a transition, the gel first collapses, then reswells, if the linear polymer concentration is continuously varied. The necessary condition for the reentrant behavior of PNIPA gels was predicted in terms of the interaction parameters among the PNIPA network, the linear polymer, and water. (C) 2002 Wiley Periodicals, Inc.