Magnetic resonance (MR) images and clinical records of 20 patients with gray matter heterotopias were retrospectively reviewed to correlate MR characteristics of the heterotopias with clinical findings. On the basis of the MR images, patients were divided into three groups: those with subependymal heterotopias (eight patients), focal subcortical gray matter heterotopias (six patients), and diffuse subcortical heterotopias (six patients). Patients with subependymal heterotopias had a significantly higher prevalence of normal development than patients in the other two groups (P = .02). When all patients with gray matter heterotopias were considered, patients with thick heterotopias and those with overlying cortical gyral anomalies, which correlated with one another, had a significantly higher prevalence of developmental delay (P = .002). Patients with thick focal gray matter heterotopias had a substantially increased prevalence of motor dysfunction. In three cases, gray matter heterotopias were associated with infoldings of dysplastic cortex containing blood vessels or cerebrospinal fluid. If not properly analyzed, these anomalies can be mistaken for vascular or cystic tumors.