Malignant proliferating trichilemmal cyst.

We report a case of a metastasizing proliferating trichilemmal cyst. A 78-year-old man had multiple common and two proliferating trichilemmal cysts, one of which showed malignant transformation as evidenced by lymph node metastases. Despite surgical removal of the malignant tumor, extensive metastatic disease rapidly occurred. This case exemplifies the difficulties in diagnosis and treatment of these rare tumors and their unpredictable course.