Multiple keratoacanthomas with peculiar manifestations and course.

The case of a 60-year-old Japanese man with unique multiple keratoacanthomas is reported. The lesions were located in the perioral area, grew peripherally, and healed centrally. Although individual lesions spontaneously regressed, prompt recurrence was observed after excision. Biopsy specimens taken from early proliferative, fully developed, and regressing stages revealed keratoacanthoma. Involucrin was demonstrated in the lesions during development and resolution. The early stage showed homogeneous staining of moderate intensity in the tumor cells at the suprabasal layers, and the staining became slightly irregular and intense when the lesions were fully developed. During resolution the intensity was markedly diminished. These results suggest that preterminal squamous differentiation accelerated during development of the tumors.