Based on clinical and histological studies in 25 patients, we have confirmed that plane warts show a characteristic phenomenon of spontaneous regression totally distinct from that described in common warts. This regression developed during various treatments in nine cases, and in 16 cases, it occurred spontaneously. In all the patients, there was a sudden and systemic onset of inflammation in every flat wart. Within two to six weeks, all the warts completely involuted. Histologically, there were variable degrees of epidermal changes depending on the stage of inflammation. However, a mononuclear cell infiltration with epidermal invasion was demonstrated in every biopsy specimen. This evidence further supports the earlier concept that this regressive phenomenon of plane warts is mediated by cellular immunity. It represents a natural experimental model of rejection of tumors induced by papovavirus in humans.