Simultaneous two-color flow cytometry of lysed whole blood allows the collection of maximal information from minimal blood volumes. This method was used for prenatal diagnosis of severe combined immunodeficiency by analysis of fetal blood. The data demonstrate that flow cytometry of lysed whole blood provides a reliable tool for prenatal diagnosis of certain immunodeficiencies. Moreover, the method described seems highly suitable for immunological monitoring of preterm infants and newborns. To enable the diagnosis of more subtle immunodeficiency states in these patients, however, valid normal values for all investigated parameters need to be defined first for the respective age/weight groups.