The vertical transmission of Trypanosoma cruzi has been augmenting its relative importance as vector and transfusion-mediated transmission routes have been, and continue to be, increasingly controlled. The vertical transmission of T. cruzi cannot be prevented; but early detection and treatment of congenital infection achieve cure rates close to 100%. In Argentina, the Subprogram of Control of Pregnant Women examined 58,196 women from 13 provinces in 1997 and found a 9% seropositivity to T. cruzi. In spite of such high maternal prevalence rates of T. cruzi, only a small proportion of live newborns to infected mothers acquires the infection. The probability of vertical transmission was 1.9% (range: 0.1% to 3.5%) in surveys carried out in the '70s, and 2.5% (range: 0.7% to 10.4%) in others conducted in the '70-'80s. Other more recent studies in Argentina estimated the probability of transmission in 2.6%-6.7%, but studies from Paraguay estimated 10.5% by PCR or serodiagnosis. The microhematocrit technique is the recommended parasitologic method to detect congenital infection. Routine serodiagnosis that detects IgG against T. cruzi is only helpful after the newborn reaches 6 months of age. Detection of specific IgM using recombinant antigens and PCR constitute excellent alternatives, but their feasibility from operational and cost-effective viewpoints in affected endemic areas remains to be considered. In a longitudinal project carried out in Maternidad Nuestra Señora de la Merced in the city of Tucumán between 1992-1994, the majority of congenital cases were asymptomatic. They were diagnosed through the microhematocrit technique, but a number of cases could only be detected later as a result of the parasitological and/or serological follow-up. Of a total of 32 newborns infected with T. cruzi who were treated with nifurtimox or benznidazole, 30 had a negative microhematocrit and serodiagnosis between 6 months and 2 years post-treatment. The magnitude of congenital transmission, and its associated morbidity and mortalidad, largely justify the efforts needed to detect T. cruzi infection in the mothers and newborns. This project demonstrated that the transmission of T. cruzi can be successfully controlled at a provincial scale through a specific program inserted in the primary health care system or at the first level of attention. The congenital transmission of T. cruzi clearly represents a public health problem in areas that in the past were of active transmission, even years after being under entomologic surveillance.