We studied 24 patients with histologically verified cerebral cavernous malformations, reviewing the familial occurrence and presenting signs, symptoms, and radiographic features of the disorder. Eleven patients had no evidence of a heritable trait and had negative family histories. Thirteen patients were members of six unrelated Mexican-American families. Sixty-four first-degree and second-degree relatives were examined, and family pedigrees were established. Most relatives (83 percent) were asymptomatic; 11 percent had seizures. Magnetic resonance imaging was performed in 16 relatives (5 of whom were asymptomatic). Fourteen of the 16 studies revealed cavernous malformations; 11 studies identified multiple lesions. As compared with computerized tomography and angiography, magnetic resonance imaging was far more accurate in detecting cavernous malformations. We conclude that cavernous malformations are more prevalent than previously reported, and that a familial form of the disorder exists that is more common than expected, with a high incidence of multiple lesions and an increased frequency of occurrence among Mexican-American families. Magnetic resonance imaging is the radiographic technique of choice for the identification and follow-up of these lesions.