Cerebral cavernous malformations (CCM) consist of clusters of abnormally dilated blood vessels. Hemorrhaging of these lesions can cause seizures and lethal stroke. Three loci are associated with autosomal dominant CCM, and the causative genes have been identified for CCM1 and CCM2. We have generated mice with a targeted mutation of the Ccm1 gene, but an initial survey of 20 heterozygous mice failed to detect any cavernous malformations. To test the hypothesis that growth of cavernous malformations depends on somatic loss of heterozygosity at the Ccm1 locus, we bred animals that were heterozygous for the Ccm1 mutation and homozygous for loss of the tumor suppressor Trp53 (p53), which has been shown to increase the rate of somatic mutation. We observed vascular lesions in the brains of 55% of the double-mutant animals but none in littermates with other genotypes. Although the genetic evidence suggested somatic mutation of the wild-type Ccm1 allele, we were unable to demonstrate loss of heterozygosity by molecular methods. An alternative explanation is that p53 plays a direct role in formation of the vascular malformations. The striking similarity of the human and mouse lesions indicates that the Ccm1(+/-) Trp53(-/-) mice are an appropriate animal model of CCM.