Congenital toxoplasmosis can cause significant neurologic manifestations and other untoward sequelae. The Palo Alto Medical Foundation Toxoplasma Serology Laboratory database was searched for data on infants 0 to 180 days old, in whom congenital toxoplasmosis had been confirmed and who had been tested for Toxoplasma gondii-specific immunoglobulin G (IgG), IgM, and IgA antibodies, between 1991 and 2005. Their clinical findings were confirmed at the National Collaborative Chicago-based Congenital Toxoplasmosis Study center. We reviewed available clinical data and laboratory profiles of 164 infants with congenital toxoplasmosis whose mothers had not been treated for the parasite during gestation. One or more severe clinical manifestations of congenital toxoplasmosis were reported in 84% of the infants and included eye disease (92.2%), brain calcifications (79.6%), and hydrocephalus (67.7%). In 61.6% of the infants, eye disease, brain calcifications, and hydrocephalus were present concurrently. T. gondii-specific IgM, IgA, and IgE antibodies were demonstrable in 86.6%, 77.4%, and 40.2% of the infants, respectively. Testing for IgM and IgA antibodies increased the sensitivity of making the diagnosis of congenital toxoplasmosis to 93% compared with testing for IgM or IgA individually. IgM and IgA antibodies were still present in 43.9% of infants diagnosed between 1 and 6 months of life. Our study reveals that severe clinical signs of congenital toxoplasmosis including hydrocephalus, eye disease, or intracranial calcifications occurred in 85% infants whose sera were referred to our reference Toxoplasma Serology Laboratory during a period of 15 years. Laboratory tests, including serologic and polymerase chain reaction tests, were critical for diagnosis in the infants. Our results contrast remarkably with those of European investigators who rarely observe severe clinical signs in infants with congenital toxoplasmosis.