Mulvihill–Smith syndrome (MSS) is characterized by premature aging, multiple pigmented nevi, decreased facial subcutaneous fat, microcephaly, short stature, mental retardation and recurrent infections, however the adult phenotype of MSS has yet to be delineated. We report a 28-year-old woman with Mulvihill–Smith syndrome, who had a solid pseudopapillary cystic tumor of her pancreas at age 17 years. Her distinctive sleep pattern includes severe insomnia with disappearance of sleep spindles and K-complexes, persisting muscle tone, and loss of slow wave sleep. The clinical and neurophysiological studies are compatible with agrypnia excitata, a sleep disorder attributable to a dysfunction of the thalamo-limbic system. Brain magnetic resonance imaging and single photon emission computed tomography revealed structural and functional deficits in the dorsomedial region of the thalamus and indicated that an alteration in the thalamo-limbic system may underlie the sleep disturbances in MSS. Furthermore, the rapid and severe decline in acquired cognitive function showed the distinct cognitive impairments resembling dementia, including intellectual deficits, memory disorder and executive dysfunction. We posit that an early onset tumor, sleep disorder and cognitive decline are adult manifestations of Mulvihill–Smith syndrome. Am. J. Med. Genet. © 2009 Wiley-Liss, Inc.