In a series of 24 apparently uncomplicated pregnancies with small-for-dates newborn at or below the tenth percentile, 15 women had hemoglobin (Hb) levels 2 SD above the mean value of normal distribution in late pregnancy. Among these 15 was the only case with intrauterine death of unknown cause. In a series of 15 cases intrauterine fetal death of unknown cause, before start of labor 10 had Hb levels 2 SD above the mean, while in a series of 16 cases of late abortion where the fetus was alive until labor started, only one had a Hb level 2 SD above the mean. In three cases with serial Hb estimations, the levels were 2 SD above the mean one week or more before intrauterine fetal death. Two of these cases also had been observed during a successful pregnancy in which the Hb levels were within normal limits. It is concluded that high Hb levels during pregnancy may indicate a fetus at risk. High viscosity of the mother's blood may impede the uteroplacental circulation, casing placental infarction, growth retardation and ultimately fetal death.