Cerebral Cavernous Malformations (CCM/OMIM 604214) are vascular malformations causing seizures and cerebral hemorrhages. They occur as a sporadic and autosomal dominant condition, the latter being characterized by the presence of multiple CCM lesions. Stereotyped truncating mutations of KRIT1, the sole CCM gene identified so far, have been identified in CCM1 linked families but the clinical features associated with KRIT1 mutations have not yet been assessed in a large series of patients. We conducted a detailed clinical, neuroradiological and molecular analysis of 64 consecutively recruited CCM families segregating a KRIT1 mutation. Those families included 202 KRIT1 mutation carriers. Among the 202 KRIT1 mutation carriers, 126 individuals were symptomatic and 76 symptom-free. Mean age at clinical onset was 29.7 years (range, 2-72); initial clinical manifestations were seizures in 55% of the cases and cerebral hemorrhages in 32%. Average number of lesions on T2 weighted MRI was 4.9 (+/-7.2) and on gradient echo sequences 19.8 (+/-33.2). Twenty-six mutation carriers harbored only one lesion on T2-weighted MRI, including 4 mutation carriers, aged from 18 to 55 yr-old, who presented only one CCM lesion both on T2-weighted and on highly sensitive gradient echo MRI sequences. Five symptom free mutation carriers, aged from 27 to 48 yr-old, did not have any detectable lesion both on T2WI and gradient echo MRI sequences. Within KRIT1/CCM1 families, both clinical and radiological penetrance are incomplete and age dependent. Importantly for genetic counseling, nearly half of the KRIT1 mutation carriers aged 50 or more are symptom-free. The presence of only one lesion, even when using gradient echo MRI sequences, can be observed in some patients with an hereditary form of the disease. Incomplete neuroradiological penetrance precludes the use of cerebral MRI to firmly establish a non carrier status, even at an adult age and even when using highly sensitive gradient echo MRI. Altogether these data suggest that the hereditary nature of the disorder may be overlooked in some mutation carriers presenting as sporadic cases with a unique lesion.