Although a number of authors (1–3) have reported the performance of pneumoencephalography with minimal removal of cerebrospinal fluid, there is still some opposition to this practice, on the basis that a significant increase in cerebrospinal fluid pressure might result, with an adverse reaction in the patient. The following study was undertaken to determine whether any significant alteration in cerebrospinal fluid pressure, as measured through the lumbar puncture needle, occurs when virtually no cerebrospinal fluid is removed during fractional air encephalography. Material and Technic Seventy-seven pneumoencephalograms were obtained on 73 patients. Eight patients had pituitary tumors extending above the sella, 9 had other intracranial tumors, 1 had undergone removal of a meningioma several years previously, 1 had a brain abscess, and 15 showed enlarged ventricles, or enlarged cerebral sulci, without evidence of ventricular obstruction (“atrophy”). The remaining patients were investigated for seizures, pos...