The authors developed a new radioisotope technique to measure placental blood flow for early detection of placental insufficiency. Using this method placental perfusion has been measured in 20 healthy pregnant women and in 15 pregnancies complicated with intrauterine growth retardation (IUGR). The T-maximum pictures obtained made it possible to differentiate between the vascular and intervillous phases of placental blood flow. The time period of intervillous phase calculated as the percent of the whole placental T-maximum was given as the intervillous perfusion index (IPI). It has been demonstrated that IPI is significantly longer in IUGR pregnancies (67.0 ± 14.6) than in the control group (31.6 ± 10.7). These data suggest that the first sign of placental insufficiency is the prolongation of IPI, which is likely to precede the quantitative reduction of placental perfusion.