158 pneumoencephalographies of infants less than 2 years old were examined and 53 of them had a large cavum veli interpositi. A neuroradiological description of this cistern is given and a comparison made with injected anatomical specimens. A statistical comparison with 105 pneumoencephalographies without a cavum veli interpositi does not shown any correlation between the presence of this cistern and pathological conditions. This investigation indicates that a large cavum veli interpositi in the newborn has no relation to hydrocephalus or cerebral atrophy. The younger the child, the more frequent is the presence of this cistern, especially under eight months of age.