Background: In familial intracranial aneurysms there is evidence for genetic h eterogeneity, probably from mutations at separate loci. Objectives: To compare d emographic and clinical features in patients of families with familial intracran ial aneurysm and different patterns of inheritance; and to compare the ages of p atients with subarachnoid haemorrhage (SAH) in affected parent child pairs to d etermine whether there is anticipation. Methods: Pedigrees for 53 families with familial intracranial aneurysms were constructed, divided into patterns of inher itance suggestive or not suggestive of autosomal dominant transmission. Demograp hic and clinical features were compared. The age at time of SAH in affected pare nt child pairs was compared using the Wilcoxon test. Results: No differences in demographic or clinical features were found between families compatible with an autosomal dominant pattern of inheritance and those with a non dominant patter n. In families with affected members in two successive generations the age at ti me of SAH in parents was 55.2 years and in children 35.4 years (mean difference, 19.8 years, p < 0.001). Conclusions: Phenotypes are similar in families with an d without a probable autosomal dominant pattern of inheritance. Thus in future g enetic studies on familial intracranial aneurysms, stratification according to p henotype is not likely to be useful. Anticipation probably occurs, as affected p arents are significantly older at the time of SAH than their affected children. Background: In familial intracranial aneurysms there is evidence for genetic h eterogeneity, probably from mutations at separate loci. Objectives: To compare d emographic and clinical features in patients of families with familial intracran ial aneurysm and different patterns of inheritance; and to compare the ages of p atients with subarachnoid haemorrhage (SAH) in affected parent child pairs to d etermine whether there is anticipation. Methods: Pedigrees for 53 families with familial intracranial aneurysms were constructed, divided into patterns of inheritance suggestive or not suggestive of autosomal dominant transmission. Demograp hic and clinical features were compared. The age at time of SAH in affected pare nt child pairs was compared using the Wilcoxon test. Results: No differences in demographic or clinical features were found between families compatible with an autosomal dominant pattern of inheritance and those with a non dominant patter n. In families with affected members in two generations of the age at ti me of SAH in parents was 55.2 years and in children 35.4 years (mean difference, 19.8 years, p <0.001). Conclusions: Phenotypes are similar in families with an d without a probable autosomal dominant pattern of inheritance. Thus in the future g enetic studies on familial intracranial aneurysms, stratification according to p henotype is not likely to be useful. Anticipation may not likely to be useful.