Antlers are periodically replaced cranial appendages that, except for the reindeer, are grown only by male deer. The annual antler cycle is controlled by seasonal fluctuations of sex steroid concentrations in the blood, and accordingly castration of male deer causes deviations from normal antler growth. The present study investigated antler histology of castrated fallow bucks (Dama dama). Castration in early spring was followed by casting of the hard antlers carried by the bucks and the growth of a new set of antlers, which remained in velvet permanently. In the following year, numerous bony protuberances developed from the original antler surface. Further growth of these protuberances, which were formed by subperiosteal intramembranous ossification, led to a marked increase in antler diameter in the affected areas. Compared to antlers of intact bucks, the antlers of the castrates showed histological signs of immaturity, suggestive of a reduced bone remodeling and an impairment of the mineralization process. These changes point to the dependence of the above processes on a stimulation by higher levels of sex steroids. Two years after castration, the antlers also developed integumental thickening and showed an initial formation of skin outgrowths. Cystic structures were present in the skin, which were often filled with a presumably sebaceous and/or keratinous material. Formation of intradermal bone or cartilage was not observed in the antlers of the castrated fallow bucks. The histological structure of the skin outgrowths suggested that they were caused by a hypertrophy of the dermal component of the velvet. Due to the localized bone overgrowth, resulting from the periosteal bone apposition onto the original antler surface, skin-lined infoldings originated, which reached deep into the newly formed bone. Our study revealed no indication of invasive/destructive bone growth in the antlers, i.e., of a penetration of the newly formed bone tissue into the pre-existing bone. The hypertrophic bone growth in the antlers of the castrates is compared with other forms of periosteally derived hypertrophic bone formation, including osteomas, in the mammalian skeleton. It is discussed whether the skin and bone outgrowths of the antlers of castrated fallow bucks may be classified as benign tumors. © 2004 Wiley-Liss, Inc.