The paper deals with the methodological problems of pneumoencephalography, with clinico-morphological assessment of detected changes and also with their possible causal role in epilepsy. A purposeful examination reveals that 30% of patients have pathological changes in the cerebrospinal fluid spaces anatomically connected with the temporal lobes and adjacent structures. The morphological substrate of these changes is not quite clear, however they resemble those seen during operations for removing epileptogenic foci. In the overwhelming majority of cases, the most frequent alterations detected by the standard PEG technique (arachnoiditis, hydrocephalus) can not be cogently attributed to epilepsy per se.