DNA deletion studies using 5 DNA markers localized at 15q11-q12 were performed in 14 Angelman syndrome (AS) patients (9 sporadic and 5 familial cases). A one-copy density for one or more of the 5 loci was detected in 8 (57.1%) of the 14 patients. A deletion of only the D15S11 locus was detected in one sporadic patient, that involving only the D15S10 in 3 familial patients (sibs in a family), that spanning 3 loci (D15S11, D15S10, D15S12) in one sporadic patient, and that spanning 4 loci (D15S9, D15S11, D15S10, D15S12) in the other 3 sporadic patients. The deletion common to our patients as well as to the reported patients may be confined to a segment between D15S11 and D15S10, if the 5 loci are ordered as cen-D15S18-(D15S9-D15S11-D15S10)-D15S12-qt er. This site overlaps but is more distal to the common deletion site in Prader-Willi syndrome (PWS) patients. In the family of the 3 sibs, both of the phenotypically normal mother and maternal grandfather also have deletions of the D15S10 locus. These results were consistent with the genomic imprinting hypothesis for the occurrence of AS, i.e., the lack of a maternally derived locus leads to AS, but may not support a model that AS is the alternative phenotype of PWS at the identical locus.