Generalized pustular psoriasis (GPP) is a serious dermatological disease characterized by fever, chills, rigors, and generalized pustule formation on the skin. Previous analyses in Japan have led to the proposal to divide GPP into two groups, one with a history of ordinary psoriasis (pso(+) GPP) and the other without a history of psoriasis (pso(-) GPP). Clinically the onset of the pustular outbreak is earlier in pso(-) GPP, which occurs more frequently after infections, whereas pso(+) GPP occurs more frequently following corticosteroid therapy. Substantial differences are also noted in HLA analyses. Activation of neutrophils is a basic mechanism in both types of GPP. Although the epidermal structural changes in GPP are usually not so prominent as those in psoriasis vulgaris, pso(+) GPP shows a more psoriasiform architecture than pso(-) GPP. Analysis of epidermal cell proliferation in GPP indicates that it is not less than that seen in psoriasis vulgaris. The occasional psoriasiform epidermal architecture especially seen in pso(+) GPP may be considered to be a steady-state condition achieved after epidermal cell proliferation has continued for a sustained period. Various inflammatory cytokines appear to be involved in the neutrophilic infiltrate seen in GPP.