Malignant pilomatrixoma in an old english sheepdog.

A tumor of hair follicle origin with histological features characteristic of a benign pilomatrixoma was removed surgically from the tail of a five year old male Old English Sheepdog. The tumor recurred locally two months after surgery, and 14 months later metastatic involvement of the lungs became clinically evident. This aggressive local growth and metastasis indicate that canine pilomatrixoma, although normally considered benign, may have malignant potential which is difficult to predict histologically.