The strain energy release rate G of a through-thickness crack in a thin film that adheres to a rigid substrate is shown to vary linearly with the film thickness at constant film stress. G is normally small, so adhered polymer films are only expected to crack in the presence of an aggressive environmental agent unless the polymer is very brittle. A minimum film thickness for cracking is likely to be observed. The propagation of crack-like defects in a polyimide in the presence of xylene was examined experimentally. The defects grew at a constant rate (independent of their length) that increased rapidly with film thickness. The minimum film thickness for defect growth was found to be about 2��m. �� 1990 Chapman and Hall Ltd.