Our objective was to determine the natural history and prognostic factors of familial forms of cerebral cavernous malformations (CCM). Cavernomas are one of the most common central nervous system vascular malformations. Familial CCM is increasingly diagnosed, but little is known about its natural history. In a national survey, we analysed clinical and MRI features of 173 patients from 57 unrelated French families. Of these 40 had undergone at least two clinical and MRI examinations. Occurrence of haemorrhage, new lesions, change in signal intensity and size of lesions have been studied by comparison between first and last MRI studies. The CCM were classified according to Zabramski et al. Mean follow-up was 3.2 years (range 0.5-6.5 years). We followed 232 cavernomas (mean 5.9 per patient, range 1-17). Serial MRI demonstrated changes in 28 patients (70%). Bleeding occurred in 21 lesions (9.1%) in 14 patients (35%). The haemorrhagic risk was 2.5% per lesion-year, higher in type I and brain-stem CCM. We saw 23 new lesions appear in 11 patients (27.5%), with an incidence of 0.2 lesions per patient year. Signal change was observed in 11 patients (27.5%), in 14 lesions (6%), while 9 lesions (3.9%) in 9 patients (22.5%) changed significantly in size.