OBJECTIVE: High-resolution magnetic resonance imaging was used to evaluate the prevalence of the cavum septi pellucidi (CSP) in 79 normal subjects and 86 patients with schizophrenia. METHOD: The CSP was assessed by counting the number of consecutive coronal 1-mm slices containing the CSP. A CSP equal to or greater than 6 mm in length was defined as large. RESULTS: The CSP was found in 74.4% of the patients and 74.7% of the normal subjects, a nonsignificant difference. No difference between groups was found in the prevalence of a large CSP. CONCLUSIONS: The findings support the idea that a small CSP is a normal anatomical variant. More cases of a large CSP are needed to elucidate the implications of this abnormality in schizophrenia.