The reported prevalence of cavum septum pellucidum (CSP), is extremely variable (from 0.1% to 85%) depending upon the measurement method or imaging resolution. Higher prevalence of CSP has been found in schizophrenia. In this study, we examined the prevalence of CSP in a large number of first-episode schizophrenia patients, young relatives of schizophrenia patients and healthy controls. We manually measured CSP using 1.5 mm T1 MRI scans from ongoing studies at University of Pittsburgh in 89 first-episode patients with schizophrenia (age=23.8+/-7.4, M/F=61/28), 64 genetically at-risk individuals (offspring and siblings of schizophrenia patients, age 15.2+/-3.7, M/F=29/32) and 120 comparison subjects (n=120, age=22.1+/-7.9, M/F62/50). CSP was present in 64% of the first-episode patients (mean length 1.87+/-2.3 mm), 64.6% of the at-risk individuals (1.64+/-1.96 mm) and 64.2% of the normal controls (1.88+/-2.0 mm). There was no difference in the prevalence of CSP exceeding 4 mm. We also did not find any influence of the sex or age in the presence or size of CSP. Our data cast doubt on the significance of CSP as markers of neurodevelopmental pathology in schizophrenia.