Several recent studies have reported an association between midline cerebral malformations (e.g., corpus callosum, cavum septum pellucidum) and schizophrenia. The authors investigated whether absence of the adhesio interthalamica (AI), a midline structure that develops in concert with prominent features of the ventricular system soon after the bridge from the late embryonic stages to early fetal life, might constitute a marker of early developmental neuropathologic changes in schizophrenia. Eighty-two patients (54 men, 28 women) with a diagnosis of first-episode schizophrenia (FES) were recruited from consecutive admissions to a psychiatric inpatient service. Fifty-two healthy control subjects (30 men, 22 women) were recruited and matched to the patient sample on distributions of sex and age. Magnetic resonance imaging studies were performed, and the presence versus absence of the AI was determined for each subject. The length and volume of the third ventricle were measured for each subject. The AI was found to be absent more often among patients with FES compared with control subjects, and patients without an observable AI also had larger third-ventricle volumes. These differences in presence or absence of the AI observed in vivo (but not in a comparable postmortem sample of histologically fixed and prepared brain slices), which are likely related to third-ventricle enlargement, may represent yet another early developmental marker of cerebral malformation among patients with FES.