We report a case of symptomatic postoperative pneumocephalus after lumbar decompression. A 69-year-old man was operated on for a severe lumbar stenosis with a L2–L4 arthrodesis and a spinal decompression. No cerebrospinal fluid leakage was visible but one of the two aspirative drains was accidentally disconnected in recovery room. After 1 day, computed tomography was performed to explore intense lumbar pain and revealed a voluminous pneumorachis. Then, the patient experienced a generalized tonic-clonic seizure. Imaging revealed a voluminous pneumocephalus responsible for a significant space-occupying effect on the frontal lobe. A conservative treatment was initiated, including bed rest, oxygen therapy, neurological monitoring and anti-epileptic therapy. Symptoms gradually improved and he was discharged without any deficit after 10 days. A total radiological regression was noted in 21 days. Prevention of postoperative pneumocephalus should include a systematic repair of iatrogenic dural tear. Even in presence of severe symptomatic manifestations, a conservative treatment is possible.