Distinction of hydatidiform moles from nonmolar specimens and their subclassification as complete hydatidiform mole (CHM) versus partial hydatidiform mole (PHM) are important for clinical practice and investigational studies to refine ascertainment of risk of persistent gestational trophoblastic disease which differs among these entities. Immunohistochemical analysis of p57 expression, a paternally imprinted maternally expressed gene on 11p15.5, and molecular genotyping are useful for improving diagnosis. CHMs are characterized by androgenetic diploidy, with the loss of p57 expression owing to lack of maternal DNA. Loss of p57 expression distinguishes CHMs from both PHMs (diandric triploidy) and nonmolar specimens (biparental diploidy) which retain expression. In the process of evaluating molar specimens in our laboratory with p57 immunohistochemistry and molecular genotyping, we identified a morphologically typical androgenetic diploid CHM with aberrant diffuse p57 expression. Molecular genotyping by short tandem repeat markers and genome-wide copy number analysis by single nucleotide polymorphism array established androgenetic diploidy with retained maternal copies of chromosomes 6 and 11, with aberrant p57 expression attributable to the latter. This case, only the second reported to date, illustrates the value of combined traditional pathologic and ancillary molecular techniques for refined diagnosis of molar specimens. Specimens with morphologic features suggestive of CHM yet retaining p57 expression should be subjected to molecular genotyping to establish a definitive diagnosis because misclassification as PHM underestimates the risk of persistent gestational trophoblastic disease. We recommend use of p57 immunohistochemistry and molecular genotyping to evaluate all products of conception specimens for which there is any consideration of a diagnosis of hydatidiform mole. Genome-wide analysis has the potential to assist in localizing imprinted genes critical for determining the morphologic and behavioral phenotypes of hydatidiform moles.