John Ruskin (1819-1900) is chiefly remembered for his works on painting and architecture, and for his powerful and original prose style. In middle age, he suffered recurring episodes of delirium with visual hallucinations and delusions. At about the same time, his writing developed a disjointed polemical character, with cryptic and intemperate elements that disorientated some readers. The nature of Ruskin's 'madness' is a key to understanding his later writing career but the psychiatric explanations given by many of his literary biographers seem unsatisfactory. Ruskin left numerous clues about the illness in his diaries, correspondence and publications. It is likely that he had a relapsing-progressive neurological disorder with neuropsychiatric manifestations. It could have been a fluctuating metabolic or immunological encephalopathy, but the diagnosis that best fits the time course of his illness and the prior history of mood disorder and of migraine with aura is Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL). Whatever the pathology, its first effects on frontal lobe function may have actually enhanced Ruskin's creative energy for a long time before stepwise cognitive impairment degraded his ability to write.