The purpose of this study was to ascertain whether parental age and parental pre-conceptional exposure to various agents differentiated children with Prader-Willi syndrome (PWS) from obese children without PWS. Two groups of patients under 25 years of age were studied, 15 persons with PWS, and 13 with PWS-like symptoms. In the PWS group deletions were detected on chromosome 15q11-q13 and parents were older. The parents' occupational and recreational exposure to environmental toxins was recorded and correlated to the clinical diagnosis, genetics and behaviour characteristics. Paternal exposure to gasoline/petrol was significantly higher in the PWS group and is suggested as a possible important factor in the aetiology of PWS.