Prader-Willi syndrome (PWS) is characterized by psychomotor and growth retardation, infantile hypotonia, characteristic facies, small hands and feet, dental abnormalities, and early onset of childhood hyperphagia with consequent obesity. PWS is associated with abnormalities of chromosome 15. Approximately 75% of patients have a deletion of 15q11q13 on the paternal homologue, whereas 20-25% have inherited both chromosome 15s from the mother and none from the father, a condition known as maternal uniparental disomy (UPD). Thus, it is a lack of paternal alleles in the 15q11q13 region that results in PWS. Thick, sticky saliva is a consistent finding in patients with PWS. We have characterized salivary flow and composition in individuals with PWS. Salivary flow in patients with PWS is approximately 20% of that in controls. In addition, the salivary ions and proteins are present in increased amounts, possibly reflecting a concentration effect relative to decreased water in the saliva. Both deletion and uniparental disomy patients exhibit these findings, suggesting that the gene(s) involved are subject to imprinting.