Cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL) has typical clinical features that include stroke, migraine, mood disturbances and cognitive decline. However, misdiagnosis is common. We hypothesized that family history is poorly elicited in individuals presenting with features of CADASIL and that enquiry into family history of all four cardinal manifestations of CADASIL is superior to elicitation of family history of premature stroke alone in raising the diagnostic possibility of CADASIL. Retrospective review of family histories at presentation in 40 individuals with confirmed CADASIL was performed through structured interview in a Neurovascular Genetics clinic (182 first-degree and 242 second-degree relatives identified). Family history obtained from structured interview was compared to family history initially documented at presentation. At initial presentation, 30% of individuals were inaccurately documented to have no family history of significant neurological illness. Thirty-five per cent of patients had an initial alternative diagnosis. Initial inaccurate documentation of negative family history was more frequent in individuals with an initial alternative diagnosis. After structured interviews, 34% of 182 first-degree and 35% of 242 second-degree relatives of CADASIL patients had history of stroke (16% of first-degree relatives had stroke before the age of 50 years). Forty-three per cent of first-degree and 28% of second-degree relatives had migraine, mood disturbance or cognitive decline. A false-negative family history was commonly documented in individuals presenting with features of CADASIL and was associated with initial misdiagnosis. Restriction of family history to premature stroke alone is probably inadequate to identify affected CADASIL pedigrees.