Prader-Willi Syndrome (PWS) is characterized by obesity, polyphagia, small hands and feet, short stature, almond-shaped eyes, hypogonadism, hypopigmentation and mental retardation. Recently it has been shown that in majority of patients (70%), PWS is caused by chromosomal microdeletion within 15q11-13. FISH analysis, using molecular probe for SNRPN gene allows verifying clinical diagnosis of PWS. In order to detect 15q11-13 microdeletion in carriers, we used two color probe mixture that contains the SNRPN specific DNA probe (labeled with Rodamine) and the PML control probe (labeled with dGreen). Our analysis was performed on a child (peripheral blood) with clinical features of PWS, and a sample of cordosynthesis of its mother, during next pregnancy. Obtain results confirmed that the child is a carrier of deletion and that the fetus has no lack of specific region which can be detect with SNRPN gene probe.