Cytomegalovirus (CMV) is the most ubiquitous member of the herpes virus family and is the leading cause of congenital (vertical) infection in newborns (Fowler, Stagno, & Pass, 2003 ; Llorente, Steigmeyer, Cooper, Rivers, & Gazley, 2011; Noyola et al., 2000 ; Steigmeyer & Llorente, 2010 ). CMV is related to the group of viruses capable of causing more pernicious infectious diseases, such as chicken pox (Santos de Barona, 1998 ). Although the virus generally remains dormant, individuals whose symptoms are clinically apparent often are dramatically affected. Common symptomatic characteristics of the virus include microcephaly, jaundice, liver-spleen infections, pneumonia, cardiac anomalies, chorioretinitis, vision loss, sensory-neural hearing loss, mental retardation, and mononucleosis (Demmler, 1991 ; Kashden, Frison, Fowler, Pass, & Boll, 1998; Noyola et al., 2000 ; Pass, 2005 ; Santos de Barona). The prognosis of individuals with CMV is highly variable, and the prognosis of individuals with congenital CMV can usually be determined based on the extent of infection at birth. The purpose of this investigation is to present longitudinal results of neuropsychological evaluation of two dizygotic twin sets (one twin of each set is asymptomatic CMV-positive and the other is uninfected) who were reared in the same environment. In addition, the present findings are discussed within the context of emerging murine and other animal analogues of CMV as well as within the extant CMV literature.