SLOW-GROWING tumors involving the frontal bone, sinus and adjacent areas are not always easily differentiated. This treatise will discuss the rather rare epidermoid type of tumor, with report of a case, and will briefly differentiate other growths with which it may be confused. An epidermoid tumor, first described in the literature by Cruveilhier1 in 1829, was termed "pearly tumor" because of its highly refractive and nodular surface. Müller2 introduced the name "cholesteatoma" in 1838 because of the cholesterol crystals in the mass. Remak3 originated the hypothesis that such growths arose from embryonic epidermal rests. Heimendinger,4 Cushing,5 Skillern,6 McFarland,7 Ewing,8 Coates,9 Critchley and Ferguson,10 Love and Kernohan,11 and Bailey12 have concurred in the belief that primary cholesteatomas or epidermoids are congenital new growths coming from misplaced or aberrant epithelial tissue. Bucy13 stated that such growths may arise beneath the scalp, within the diploe, between the bone and dura mater,