Flow cytometry was used to determine lymphocyte subpopulations in maternal blood from 143 pathological pregnancies: 50 with fetal aneuploidy; 32 with missed abortions; 12 with ectopic pregnancies; 20 with multi-fetal pregnancies, and 29 with pregnancies complicated by intrauterine growth retardation (IUGR). The values were compared to those of 240 women with normal singleton pregnancies at 8–40 weeks of gestation and 20 non-pregnant controls. In early pregnancy (8–10 weeks), compared to non-pregnant values, there was a decrease in the percentage of CD4+ cells and CD4+ to CD8+ ratio and an increase in the percentage of CD8+ cells. In later pregnancy, the CD4+ cell percentage and CD4+ to CD8+ ratio increased and the CD8+ cell percentage decreased to reach non-pregnant values at term. The percentage of natural killer (CD3–– and CD 16/56+) cells decreased with gestation, while the percentage of B (CD 19+) cells did not change significantly. In IUGR, the percentage of CD4+ cells and CD4+ to CD8+ ratio were decreased, while the percentage of CD8+ cells was increased. In contrast, in the groups of missed abortions and ectopic pregnancies, the CD4+ to CD8+ ratio was increased. In multifetal pregnancies and those with fetal aneuploidies there were no significant differences in maternal lymphocyte subpopulations from