&NA;Although the advent of computerized cranial tomography (CT) has decreased the number of pneumoencephalograms performed for the diagnosis of hydrocephalus and lesions of the posterior fossa, brain stem, and ventricles, there are some patients in whom pneumoencephalography should still be done because it adds valuable information to that obtained with CT. When the temporal horn becomes obstructed, the choroid plexus and ependymal surface “upstream” from the obstructing mass continue to produce cerebrospinal fluid (CSF). The temporal horn can thus enlarge enough to appear as a mass on CT because of its reduced x‐ray attenuation coefficient. Pneumoencephalography is effective in this situation because air will flow past a mass that obstructs CSF and because the ventricular system dilates during pneumoencephalography. When pneumoencephalography is used in a patient with a trapped temporal horn, the partially trapped horn may enlarge approximately 24 hours later. With that precaution in mind, the neurosurgeon should find pneumoencephalography to be a useful adjunct to CT in delineating the cause of a trapped temporal horn. In the three patients reported here CT had indicated a unilateral trapped temporal horn; pneumoencephalography confirmed that finding and demonstrated both the location and the nature of the lesion. One patient had a Grade II astrocytoma fungating into the atrium of the right lateral ventricle, one had a mass extending into the right ventricle from the medial and superior ventricular wall with nodular encroachment on the ventricle, and one had a meningioma in the atrium of the right lateral ventricle.