Abstract: The authors made a clinical analysis of 82 cases of the unstable psoriasis (pustular and erythrodermic psoriasis). The result suggests that the course and the transition of various types of psoriasis may be like a spectrum. According to its stability, it may be divided into 3 phases: stable, unstable and pre-unstable phases. The authors postulate a mechanism of development of the unstable psoriasis. Furthermore, the authors suggest the indication and prevention of the transition from psoriasis vulgaris to pustular and erythrodermic psoriasis.