Two infants with chromosome 22q11 deletion syndrome were noted to have symmetrically enlarged Sylvian fissures on cranial MRI. We compared the size of the Sylvian fissures in neuroimaging studies from 17 other subjects with del 22q11 to age-matched disease controls. The mean anterior interopercular distance was used as an index of Sylvian fissure enlargement. Symmetric enlargement of the Sylvian fissures was present in 10 of 17 subjects with del 22q11. The age-incidence pattern, as well as follow-up scans in 2 patients, suggests delayed growth of the opercular region in these patients. Subjects with del 22q11 consistently had disproportionate enlargement of the left Sylvian fissure compared to the right. This observation suggests that a gene (or genes) in the deleted region affects the development of the left and right perisylvian cortex in different ways. Abnormal development of the operculum may explain some of the neurodevelopmental features that are common among individuals with 22q11 deletion syndrome.