Arteriovenous malformations in the posterior fossa are extremely uncommon (2, 5). In a review of the literature, Askenasy et al. (1) found that most of the reported cases were associated with hydrocephalus, as is the case described herein. In addition to recording this rare entity for the literature, a plain roentgenographic finding, to our knowledge not previously noted, is described. Case Report Clinical Course: At the time of the diagnostic admission, the child was twenty months of age. Growth and motor development had been normal following an uneventful pregnancy and delivery. Until the age of seventeen months, the mother noted only a relative enlargement of the head, but, as the child was doing well, no studies other than plain roentgenograms of the skull were made. At this admission, however, a more rapid enlargement of the head was apparent, and a vascular bruit could be detected over the occiput bilaterally. The child otherwise remained normal. Plain Radiographs: Initial skull films at eleven months of age (Fig. 1) depicted only a relative disproportion between the vault size and the facial structures, consistent with hydrocephalus. On the half-axial film at twenty months (Fig. 2, A) the groove for the right transverse sinus was markedly increased to a diameter of 33 mm compared to an upper limit of normal of about 17 mm (4). The torcular Herophili was markedly enlarged as well. The left transverse sinus was prominent, but within the range of stated normal. The lateral film (Fig. 2, B) showed a localized bulge in the region of the confluence of sinuses and right transverse sinus not present on the initial lateral studies. Arch Angiogram: A contrast study performed with the catheter tip in the ascending aorta depicted an extensive malformation with a 4 cm aneurysm (Fig, 3). This was supplied primarily through the superior and posterior inferior cerebellar arteries. A portion of the blood flow into the posterior circulation was from the carotid circulation by way of a markedly dilated posterior communicating artery. The contour of the anterior cerebral artery as well as the sylvian vessel indicated the associated marked hydrocephalus. The deep central venous pattern was not well displayed, but venous drainage from the malformation into a massively enlarged dural sinus complex was shown. The sinus outline (Fig. 4) corresponds angiographically to the plain-film display of the vascular grooves. Although the lateral projection suggests the presence of two aneurysms, one is the true venous aneurysm while the other is “aneurysmal” dilatation of the venous sinuses.