Costello syndrome is characterised by postnatal growth deficiency, coarse facies, redundant skin on the neck, palms, soles, and fingers, dark skin, acanthosis nigricans, and papillomata. The natural history evolves in two phases, a severe failure to thrive during the first months contrasting with a normal weight gain in later life. Cardiomyopathy is frequent but other visceral involvement is rare. Mild to moderate mental retardation is usual and most patients exhibit a characteristic sociable and friendly personality. The pathogenesis and molecular basis of the syndrome are unknown and the diagnosis is reliant on clinical expertise. Papillomata represent the most characteristic manifestation but may arise late in life. The peculiar course of the disease, the typical facies, and the ectodermal involvement with loose and hyperpigmented skin are characteristic enough to allow an early diagnosis. Most cases have been sporadic, suggesting de novo dominant mutations.