Prenatal diagnosis of fetal toxoplasmosis is possible with the use of fetal blood sampling, amniocentesis and ultrasound examination. The purpose of this study was to describe T lymphocyte subsets (CD3, CD4 and CD8) in mothers and their fetuses when Toxoplasma gondii infection occurred during pregnancy. Maternal and fetal blood samples were obtained in 86 cases and 9 fetuses showed T. gondii infection. Control groups consisted of 30 healthy nonpregnant women and 30 pregnant women. Pregnant women with T. gondii infection showed an increase in the suppressor (CD8) T subpopulation and a significant depression in the total helper (CD4) T cells. These alterations were more important in mothers whose fetus was infected. We showed the progressive maturation of the fetal immune system with a regular increase of all T lymphocyte subsets. Marked alterations were observed in the 9 infected fetuses (depression of CD4 population and lower CD4/CD8 ratio). In the future these differences might be used as a new marker of the severity of fetal lesions and become a useful diagnostic tool.