The authors have evaluated placental blood flow using a non-invasive radioisotopic approach. The procedure involves the intravenous administration of 113mIn and the technique is a modified version of a previously reported method for a non-diffusive tracer kinetics analysis. The method is presented in detail and its performance is evaluated in 17 normal and 32 pathologic pregnancies (19 intrauterine growth retarded patients; 7 patients with iso-Rh immunization; 5 patients with gestational diabetes, and 1 pregnancy with extensive fetal malformation which was not included in any group). The normal pregnancies showed a flow index of 5.34 ± 1.64 (1 SD) units compared to the pathological pregnancies value of 2.73 ± 0.73 (1 SD) units (p < 0.001). The conclusions drawn are: (1) the method is very well suited to clinical evaluation of placental blood flow; (2) the evaluation of placental blood flow cannot be directly equated with fetal development as it is not its only determinant, and (3) no other independent test to evaluate the radioisotopic study in terms of accuracy exists.