The behaviour of a pre-crack inserted in a surface swollen layer formed by case II diffusion of methanol in poly(methyl methacrylate) (PMMA) was investigated with reference to the distribution of internal compressive stress induced under constraint by the inner glassy core. The elastic analyses of internal stress were conducted by a finite element method using the data for the mechanical properties and the swelling strains preliminarily obtained for the sheet specimen overall swollen in methanol. The growth of the inserted crack in the surface swollen layer under static tension in methanol at 20 degrees C was almost arrested for a long period at a position where the internal compressive stress in the tensile direction took the maximum magnitude near the boundary between the swollen layer and the glassy core. After being released from the internal compressive stress, the crack rapidly progressed to lead to a general fracture going across the boundary. A relatively long crack inserted in the thinner surface swollen layer completely disappeared in methanol at 40 degrees C during 3 min under no external load. Crack healing, however, was not observed either in methanol at the lower temperature of 20 degrees C or in the entirely swollen specimen, even at 40 degrees C. These results suggest that both high temperature above the glass transition temperature of swollen PMMA and the significant internal compressive stress are required for crack healing.