In a population-based study of Prader Willi syndrome (PWS), we investigated the relation between genetic subtypes of the syndrome and psychiatric morbidity. Of 25 patients aged 18 years or older, seven (28%) had severe affective disorder with psychotic features, with a mean age of onset of 26 years (SD 5.9). The seven people affected, all aged 28 years or older, included all five with disomies of chromosome 15, one with a deletion in this chromosome, and one with an imprinting centre mutation in the same chromosome. We postulate that in PWS, an abnormal pattern of expression of a sex-specific imprinted gene on chromosome 15 is associated with psychotic illness in early adult life.