Two sibs, a 13-year-old girl and an 11-year-old girl, with typical clinical features of the Prader-Willi syndrome (PWS) are reported. High-resolution chromosome analysis showed the normal karyotype in the elder sister, and 46,X,del(X)(pter----q26.1:) in the younger sister. But an interstitial deletion of 15q was not detected in either of the cases. PWS is most probably an etiologically heterogeneous syndrome consisting of two subgroups, with partial deletion and non-deletion of chromosome 15, respectively.