In a prospective clinical study the safety of two hydroxyethylstarch preparations (HES steril 10%, Fresenius AG, Oberursel = HES-A; Haemufusin, Kabi-Pfrimmer, Erlangen = HES-B) were assessed. In 60 patients with fetal growth retardation and/or gestational hypertension, hematocrit, aPTT, factor VIIIR: Ag, fibrinogen, uric acid, cord blood hemoglobin, hematocrit, pH-value and the fetal/maternal hydroxyethylstarch concentration before and after eight (HES-B) or nine (HES-A) days of treatment were monitored. 500 ml HES-A (n = 36) or HES-B (n = 24) together with the same volume electrolyte solution, were infused daily. Both substances lowered significantly the maternal and fetal hematocrit. Histopathological changes of placenta (trophoblast cells and stroma) taking place after the infusion of HES-A or HES-B were depicted by light microscopy. Administration of HES-A or HES-B was associated with lower values of factor VIIIR: Ag and a prolongation of aPTT, but only HES-B demonstrated a significant effect (31% vs. 12%, p less than 0.01). We observed in 4 (16.7%) cases severe uterine bleeding complications and one woman (4.2%) with abruptio placentae in the group HES-B. Light microscopy shows vacuoled trophoblast and stroma cells after HES infusions. The marked vacuolisation of the placenta after HES-B is due to differences in the physicochemical characteristics of HES-A and HES-B. For this reason, we prefer to administer HES-A in the dilution treatment of patients with placental insufficiency.