Forty patients with Sturge-Weber syndrome were studied over a 26-year period. The nevus flammeus was unilateral in 27 patients (twice as often on left side) and bilateral in 13 patients. Only 3 of these 13 patients had bilateral cerebral lesions. Seizures, most of which were focal, were present in 32 patients (80%). The percentage of patients in whom the seizures subsequently became generalized was very high. Seizure presentation coincided with febrile episodes in 10 of 32 patients (31%). Total seizure control was obtained in 15 patients (47%). Abnormality of the cerebral parenchyma can be detected from birth in some patients and has a progressive character; at the same time, progressive atrophy and parenchymatous hyperdensity of the affected hemisphere is evident, as well as a decrease in arterial size, especially during the first decade of life. No relationship exists between the size of the facial nevus flammeus or its unilateral or bilateral location and clinical neurologic impairment. Conversely, a direct relationship exists between greater anatomic manifestations (i.e., atrophy, calcification) in the involved hemisphere when the lesion is unilateral as well as the presence of leptomeningeal angiomatosis in both cerebral hemispheres in patients with bilateral facial nevus flammeus and the severity of clinical disorders. Mental retardation was present in 60% of patients and was severe in 32.5% of all patients. Even though computed tomography and T1- and T2-weighted magnetic resonance imaging have great diagnostic value, magnetic resonance imaging enhanced with gadolinium-DTPA discloses the cerebral, leptomeningeal, and ocular lesions before the first evidence of neurologic abnormality.(ABSTRACT TRUNCATED AT 250 WORDS)