A subgroup of patients with West syndrome (WS) also have focal features, and some of these patients respond to surgical intervention. To determine the incidence of focal features in patients with WS and to establish the correlation of these features with outcome, all patients with WS treated in the Epilepsy Unit of the Children's Hospital (Boston, MA) during the years 1989 to 1992 were retrospectively evaluated. Focal features included partial seizures, hemiparesis, focal radiologic findings, asymmetric spasms, and lateralized hypsarrhythmia. The different focal features were later correlated with the natural history of the disease. Sixty-seven patients were included in the study. Sixty-six percent had asymmetric manifestations. Asymmetric spasms, hemihypsarrhythmia, and partial seizures combined with infantile spasms had significant association with asymmetric brain pathology. Focal features did not correlate with age of onset and outcome, but were associated with etiology. Eight of the nine patients who underwent epilepsy surgery had at least two focal features or more. Two of the operated children had focal features in the absence of magnetic resonance imaging abnormalities, thus indicating the necessity for positron emission tomography, the results of which justified surgery.