This study investigated the thickness of a swollen layer between heat-activated and autopolymerized denture base polymer based on poly(methyl methacrylate) and autopolymerized repair acrylic resin. The repair surfaces of polymerized poly(methyl methacrylate) resin specimens (n = 5) were wet with methyl methacrylate liquid for 0, 0.5, 3, 30 or 60 minutes before the mixed repair resin was app lied and polymerized to the surface of the specimen. The specimens were wet ground to a thickness of 0.15 mm, and the repair joint was examined using a light microscope. The thickness of the layer of swollen poly(methyl methacrylate) at the repair joint was measured from the micrographs. Both the type of polymer, ie, heat-activated or autopolymerized, and the wetting time of the repair surface with methyl methacrylate had a significant effect on the thickness of the swollen layer (P .001). Prolonged wetting time of the repair surface of the autopolymerized poly(methyl methacrylate) increased the thickness of the swellen layer (r = .944, P = .000), but had less effect on the thickness of the swollen layer of h eat-activated poly(methyl methacrylate) (r = .391, P = .005). This study suggests that methyl methacrylate diffuses more easily into an autopolymerized poly(methyl metha crylate) than into heat-activated poly(methyl methacrylate) during the repair of denture base polymer.