Nocturnal polygraphic sleep studies were conducted with 10 mentally retarded infants (2 months to 4 years of age ) suffering from hydrocephaly. In two cases of aqueduct stenosis with ventriculitis (A) and unknown etiology (B), it was not possible to distinguish between EEG patterns of wakefulness, sleep Stage 1, sleep Stage 2 and REM sleep because of abnormally high voltage fast activity during each of these periods. Spindles were observed in the latter case (B) but not in the former case (A). These four stages could, however, be differentiated on the basis of EOG, respiration and submental EMG patterns. One case each of postmeningitis and subdural effusion exhibited no spindles. In the remaining 6 cases, the EEG patterns characteristic of the sleep-wakefulnss cycle were observed. In one of these cases (postmeningitis), extreme spindles were evident. A periodic rhythmicity in the integrated delta levels was found in all cases.