Effects of sorbed water on crack propagation in poly(methyl methacrylate) (PMMA) under static tensile stress have been investigated. The specimens were kept for more than two years in temperature and humidity-controlled conditions. Sorbed water of less than 0.40 wt% scarcely affected K-Ith value (threshold stress intensity factor for crack propagation), however, K-Ith value for the specimens containing water of more than 0.40 wt% increased with the amount of sorbed water. K-Ith values related to the balance among the radius of the curvature of crack tip, crazing stress and craze fibril rupture stress, which are functions of the amount of sorbed water. At a crack propagation rate of more than 1 x 10(-7) m/s, the slopes of K-da/dt curves for the specimens containing water less than 0.40 wt% were gentle, however, that for the specimens containing more than 0.40 wt% was steep; and unstably fractured. It was found that the gentler slopes for the specimens containing little sorbed water may be caused by craze-shear controlled crack propagation mechanisms, while the steeper slopes for the other specimens may be caused by a craze controlled crack propagation mechanism.