Serum erythropoietin (Epo) concentration was compared prenatally in adequate (AGA) and small for gestational age (SGA) fetuses. Fifty-four percutaneous umbilical blood samplings were paired with maternal blood and assessed for Epo, hematocrit and reticulocyte count. Seventeen fetuses were growth-retarded (SGA) on the basis of ultrasonic biometry and birth weight. Controls (AGA; n = 37) underwent cordocentesis for risk of toxoplasmosis, maternal age or malformations (CNS, GI, heart). No cytogenetic abnormality or infections was found in the 54 fetuses. Linear and polynomial regressions were fitted to determine correlations between parameters. Nonparametric Mann-Whitney test was used for comparison between groups. Gestational age at sampling was similar in AGA and SGA. The SGA fetuses had a higher Epo concentration than controls (p < 0.001). Fetal AGA Epo increased significantly throughout pregnancy (p < 0.01) but did not correlate with maternal Epo (p > 0.05). An inverse correlation was found between fetal reticulocyte count and Epo concentration (p < 0.02). The mechanism of high Epo level in SGA fetuses may involve low pO2 and hemoconcentration.