Prader-Willi syndrome (PWS) is a genetic disorder caused by the loss of the paternal contribution of the proximal part (15q11-q13) of the long arm of chromosome 15 (i.e. deletion, disomy and imprinting mutation). The syndrome is associated with distinct physical dysmorphism, as well as with specific behavioural and psychopathological characteristics. Psychiatric symptoms in adolescence and adulthood have been described, including acute cycloid psychosis, and obsessive compulsive, bipolar and pervasive developmental disorders. At the Centre for Human Genetics in Leuven, Belgium, 53 individuals (31 children and adolescents, and 22 adults) have been followed up for 15 years by a special multidisciplinary team. Attention was given to their medical, cognitive, behavioural and emotional development, and the evolution of psychiatric disorders in adolescence and adulthood. This study describes the psychiatric problems in four patients diagnosed with acute cycloid psychosis and traces their development from infancy to adolescence. Four other individuals needed psychiatric evaluation and treatment, and could be diagnosed as having unspecified bipolar disorder, also termed unstable mood disorder. Both groups were compared, and significant differences in early development and later evolution into adulthood were noted. The individuals with PWS who later developed psychotic episodes were described as active and extrovert toddlers, and showed autistic behaviour during their primary school education. Their intellectual functioning was in the moderate to severely retarded range. The individuals with PWS who later developed an unstable mood disorder were described as rather passive and introvert toddlers, and they presented less disturbed behaviour during their primary school education. The intellectual functioning of these subjects was in the normal to borderline range.