Thermally reversible macroporous poly(N-isopropylacrylamide) (polyNIPAAm) hydrogel has been synthesized in aqueous solution at a temperature above the lower critical solution temperature (LCST). Rapid swelling and deswelling kinetics of this macroporous hydrogel are characterized. Pore size measurement by a solute exclusion technique reveals that this macroporous hydrogel has a larger pore size distribution than the hydrogel synthesized at a temperature below the LCST. This kind of thermally reversible macroporous hydrogel will be very useful in delivery of macromolecules.