Leptomeningeal carcinomatosis is rare in evolution of head and neck cancers. Prognosis is poor and treatment is through supportive care. We report an atypical rapid post-operative leptomeningeal carcinomatosis after skull base resection for an ethmoidal adenocarcinoma. The anterior skull base resection was near-total and we suspect a seeding of subarachnoid space either before surgery, either during surgical extirpation or from the residual intracranial nodule. It is the first description of rapid LMC after surgery (less than 1 month). Post-operative MRI diagnosed the LMC through the detection of contrast enhancing leptomeningeal nodules at bilateral second and eighth cranial nerves and at bilateral interpedoncular area. A strong index of suspicion along with clinical and radiological correlation is essential to arrive at a correct diagnosis. Otolaryngologists should be aware of the possibility of leptomeningeal carcinomatosis, before or after surgery, in a patient presents with an appropriate constellation of neurologic symptoms and a history of cancer.