In vitro release of ketotifen from transdermal systems made of silicone pressure-sensitive adhesive matrices (PSA) containing crystalline ketotifen (PSA-Crystalline) and that containing amorphous ketotifen (PSA-Amorphous) were analyzed using two types of mathematical models. Higuchi's model, which assumes an infinite drug dissolution rate in the matrices, well described the release from PSA-Amorphous. While Higuchi's model overestimated the release from PSA-Crystalline, the deviation was significantly diminished by incorporating a dissolution control term into the model. This analysis suggests that ketotifen release from PSA-Amorphous is almost entirely diffusion-controlled, while that from PSA-Crystalline is controlled by both diffusion and dissolution processes.