A molecular-genetic study was performed in 61 patients with Angelman syndrome (AS) and 14 patients with Prader-Willi syndrome (PWS). Southern blot analyses and/or PCR-mediated dinucleotide repeat polymorphism (DNRP) analyses revealed that 67% of AS patients have DNA deletions ranging from D15S9 to D15S12 loci. An exception was 3 sib cases whose deletion involved only 2 loci, D15S10 and GABRB3. The parental origin of the deletions in AS patients were exclusively maternal. No uniparental disomy (UPD) was found in our AS patient series, suggesting that UPD in AS is infrequent than that in PWS. Molecular deletions were observed in 6 of the 14 PWS patients. In order to develop a simple, reliable DNA-based diagnostic method, I adopted PCR-mediated DNRPs as genetic markers for the detection of deletions and/or parental origin of chromosomes 15 in AS and/or PWS patients. This method gave useful diagnostic information in 33 (89%) of 37 AS patients and 12 (86%) of 14 PWS patients, indicating no big difference from the information obtained with Southern blot analysis. Furthermore, since this DNRP method dose not require radioisotopes, it may be a first-choice, alternative way when diagnosing AS or PWS patients.