We studied proliferating pilar tumors (PPTs) to establish histologic criteria that could predict behavior. We reviewed all cases in our consultation files (1989-2000) and evaluated 76 cases with meaningful follow-up information. Histologic examination involved attention to tumor silhouette, degree of nuclear atypia, mitotic activity, necrosis, and perineurial or angiolymphatic invasion. Tumors were stratified as follows: group 1 PPTs, circumscribed silhouettes with "pushing " margins, modest nuclear atypia, and an absence of pathologic mitoses, necrosis, and invasion of nerves or vessels; group 2 PPTs, similar to group 1 but manifested irregular, locally invasive silhouettes with involvement of the deep dermis and subcutis; group 3, invasive growth patterns, marked nuclear atypia, pathologic mitotic forms, and geographic necrosis, with or without involvement of nerves or vascular structures. Recurrence occurred in none of 48 group 1 PPTs; 3 (15%) of 20 group 2 PPTs had local regrowth; 4 (50%) of 8 of group 3 PPTs recurred and/or metastasized to regional lymph nodes. The differences between groups 1 and 3 and between 2 and 3 were statistically significant (P = .0002, P < .05, respectively). It seems justifiable to regard group 1 PPTs as benign, group 2 as having potential for locally aggressive growth, and group 3 also as having metastatic potential. The latter 2 categories might be equated with low and high grades of malignancy among PPTs of the skin.