We report on the second trimester prenatal diagnosis of an apparently nonmosaic tetraploid fetus, 92,XXYY. Indications for cytogenetic studies of the fetus included abnormal ultrasound findings and abnormal maternal serum levels of alpha-fetoprotein (AFP)/estriol. Chromosome analysis of amniocytes documented tetraploidy, a finding confirmed by flow cytometry of several fetal tissues. Autopsy findings in the fetus are compared with those of other cases of tetraploidy. To our knowledge this is the first reported prenatal diagnosis of a tetraploid fetus. Additionally, it illustrates the value of flow cytometric analysis of products of conception in which polyploidy is suspected.