BACKGROUND: Melanoma characteristically grows within the epidermis along the dermal-epidermal junction, sometimes extending outward up to several centimeters beyond the foci of invasive tumors. Although follicular involvement by malignant melanoma is widely recognized, to the authors' knowledge no previously published data address this phenomenon. METHODS: To examine the growth characteristics of in situ melanomas in relation to the hair follicle microanatomy, the authors analyzed 100 cases of primary cutaneous melanomas (61 in situ and 39 invasive melanomas with significant in situ components) obtained from pathology clinical archives. RESULTS: Eighty-two (82%) cases of melanoma in situ demonstrated tumor cells within >or=1 hair follicles. Of those, 57 (69.5%) cases demonstrated the tumor cells only within the infundibulum. Extension of the tumor cells down to the isthmus was observed in 24 cases (29.3%). In only 1 exceptional case (1%) were tumor cells detected beneath the level of the hair follicle bulge. CONCLUSIONS: The authors postulate that a physiologic barrier restricts the intraepithelial spread of melanoma tumor cells at or beyond the level of the stem cell niche in the hair follicle bulge. Although the nature of this barrier remains to be elucidated, the distinct biologic characteristics of the hair follicle bulge may provide clues to understanding this phenomenon.