Here we report on an infant of a diabetic mother (IDM) with midline interhemispheric "fusion" (MIF), or syntelencephaly. This is a rare anomaly characterized by segmental failure of cleavage of the cerebral hemispheres and other brain structures in the posterior frontal and parietal regions, with a normal interhemispheric fissure anterior and posterior to the "fused" region. While there is obvious overlap with holoprosencephaly (HPE), this condition differs from HPE in that the midline "fusion" in MIF is complete but segmental, while the structural brain anomalies seen in the HPE spectrum progress smoothly in severity in a posterior to anterior "fusion." However, while it is apparent that there are key distinctions between MIF and HPE, in all likelihood they arise from a similar pathogenetic mechanism. We therefore suggest that MIF is a distinct variant of the HPE spectrum of midline brain anomalies. Given the known increased incidence of HPE in IDMs, MIF is likely a maternal diabetesassociated malformation. © 1996 Wiley-Liss, Inc.