In a clinical and cytogenetic survey of 39 individuals with Prader-Labhart-Willi syndrome (PLWS) (23 males and 16 females ranging in age from 2 weeks to 39 years), an interstitial deletion of chromosome 15 (breakpoints q11 and q13) was identified in 21 cases and apparently normal chromosomes in the remainder. Studies of parental chromosome 15 variants showed that the del[15q] was paternal in origin, although chromosomes of both parents were normal. All chromosome deletions were de novo events. Possible causes for the chromosome deletion and the role of chromosome rearrangements in individuals with PLWS are discussed. Clinical characteristics of the deletion and nondeletion groups were recorded and compared with 124 individuals reported in the literature. Individuals with the chromosome deletion were found to have lighter hair, eye, and skin color, greater sun sensitivity, and higher intelligence scores than individuals with normal chromosomes. Correlation studies of metacarpophalangeal pattern profile variables and dermatoglyphic findings indicate apparent homogeneity of the deletion group and heterogeneity of individuals with PLWS and normal chromosomes.