The clinical phenotype in cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL), an autosomal dominant cerebral arteriopathy, is variable, but the reasons for this remain uncertain. Possible factors include the mutation site and the influence of additional modulating factors, which could include both epistatic interactions and interactions with cardiovascular risk factors known to cause sporadic small vessel disease. In a large prospectively recruited cohort of CADASIL subjects we determined relationships between phenotype and mutation site, the apoE genotype and cardiovascular risk factors. In addition to clinical features, disease severity was assessed by MRI lesion volume, measured both semiquantitatively (Scheltens scale) and quantitatively. One hundred and twenty-seven CADASIL cases from 65 families with 17 different mutations were studied. Site of mutation was not associated with the presence or age of onset of stroke, migraine, dementia, dependency or MRI lesion load. There was no evidence of intrafamilial clustering of particular phenotypes. Amongst subjects with stroke/transient ischaemic attack, smoking at the time of the event was independently associated with earlier age of onset (P = 0.01). There were no associations between age of onset or presence of stroke and other cardiovascular risk factors, including homocysteine. Homocysteine levels were higher in migraineurs [mean (SD) 12.8 (5.6) versus 9.8 (3.4) micromol/l, P = 0.02)] and elevated homocysteine was independently associated with an earlier age of onset of migraine (P = 0.01). No relationship was found between MRI lesion volume and risk factors, or between apoE genotype and phenotype. Our results show no notch 3 genotype-phenotype correlations. This implies that modulating factors influence phenotype. Smoking appears to increase the risk of stroke, while high homocysteine levels are associated with an increased risk of migraine.