OBJECTIVE: Cavernous malformations are angiographically occult cerebrovascular malformations found in approximately 0.5% of the population. To help further understand the natural history of these lesions, we prospectively followed 68 patients harboring cavernous malformations. METHODS: The 68 patients in this study were all diagnosed radiographically (67 patients) or surgically (1 patient) and were entered into a patient database. Age, sex, clinical symptoms, seizure frequency, focal neurological deficits, and presence or absence of extralesional hemorrhage were all recorded at presentation. Patients were then followed prospectively to determine the rate of hemorrhage and new-onset seizures. RESULTS: The mean follow-up per patient was 5.2 years, and the total follow-up was 352.9 patient-years. There was an average of 3.4 lesions per patient. Thirteen of the patients (19%) had familial cavernous malformations. Patients with familial disease were more likely to have multiple lesions than patients with sporadic disease (85% versus 25%, respectively [P = 0.001]). Initial presentation included headache (65%), seizures (49%), and focal neurological deficit (46%). Eleven symptomatic, radiologically proven, extralesional hemorrhages occurred during the 352.9 patient-years of follow-up for an overall hemorrhage rate of 3.1% per patient-year. Female patients had a significantly higher prospective hemorrhage rate (4.2% per patient-year versus 0.9% per patient-year [P = 0.04]). A history of hemorrhage at presentation was not a risk factor for subsequent hemorrhage during follow-up. The rate of new-onset seizures was 2.4% per patient-year. CONCLUSION: The clinical presentation and prospective hemorrhage rate reported here agree well with findings of other prospective studies. This information, combined with our new-onset seizure rate, should aid clinicians caring for patients with cavernous malformations.