A ventricular diverticulum was found on computed tomography in six out of 25 cases of congenital obstructive hydrocephalus. In all six cases, the diverticulum was located on the medial wall of the trigone. In three cases, it was restricted to the tentorial hiatus with a recognisable superior cerebellar cistern (small). In two cases, it occupied the tentorial hiatus without a recognisable cistern (medium). The remaining case had a diverticulum extending into the posterior fossa that caused cerebellar ataxia (large). In all patients, the diverticulum regressed after ventriculo-peritoneal shunt. The characteristics of the computed tomograms, the clinical significance and the pathogenesis of this phenomenon are discussed.