Studies of congenital toxoplasmosis in twins confirm the definite role of the placenta in the modalities and mechanism of fetal contamination. In single-chorion twin pregnancies, clinical manifestations are generally identical in both infants. Conversely, twins from double-chorion pregnancies usually have different clinical patterns; occasionally, only one of the twins is affected (1 case). The diagnosis can be ascertained antenatally by sampling blood from each of the fetuses (2 cases). The cases reported herein illustrate some of the diagnostic pitfalls that may lead to inappropriate discontinuation of monitoring and treatment: negative placental studies, absence of specific IgM antibodies, transient fall in IgG antibody titers, delayed fetal contamination after a negative fetal blood study, and need for routine tests for increased CSF albumin levels.