The olfactory cleft is a narrow chamber located under the cribriform plate and between the turbinate wall of the ethmoidal labyrinth and the corresponding nasal septum. Nasal adenocarcinomas are mostly described as originating in the ethmoid sinus and operated via external approaches. We designed a prospective study on twenty consecutive woodworkers' adenocarcinomas without intracranial extension to determine the precise site of origin of the tumour. All patients were operated under endoscopic endonasal control according to a methodical surgical procedure as follows: 1) debulking of the tumour and identification of the middle turbinate or conchal lamina, 2) exenteration of the ethmoidal labyrinth according to the nasalisation procedure, and 3) exenteration of the olfactory cleft. Endoscopic endonasal surgery showed that woodworkers' adenocarcinomas constantly originated in the olfactory cleft, appearing as polyp-like neoplasms with well-defined bodies. Over a long period of time, they do not invade, but just displace and push out the surrounding structures, i.e. the nasal septum and the turbinate wall. More than the volume of the tumour, the precise location of the pedicle and especially its connection to the cribriform plate could be of major prognosis value.