Serum immunoreactive erythropoietin (siEPO) was determined in cord serum from neonates (n = 97, gestational age 36-43 weeks), in healthy children from birth to adolescence (n = 260) and in children with haematological (n = 30), renal (n = 10) and congenital heart diseases (n = 70). In healthy children siEPO levels decreased after birth (geometric mean cord siEPO 35.6 mU/ml with 95% range of 17-56 mU/ml in eutrophic, nondistressed fetuses) and reached lowest values during the first 2 months (geometric mean siEPO 11.5 mU/ml). Thereafter siEPO levels increased slightly and were constant between 2 months and adolescence. The geometric mean siEPO for healthy children after birth was 18.8 mU/ml with 95% range of 7-47 mU/ml. These estimates were not significantly different from normal adult values. In newborns with fetal distress (n = 15) cord siEPO was significantly elevated (geometric mean 63.0 mU/ml; P less than 0.001). In children with haematological disease, siEPO and Hb concentration were inversely correlated (log siEPO (mU/ml) = 4.1-0.20 x Hb (g/dl); r = -0.62; P less than 0.0005). This relationship was significantly different in children with chronic renal failure (log siEPO (mU/ml) = 0.67 + 0.035 x Hb (g/dl); r = 0.50; P = 0.1). In children with heart disease the geometric mean siEPO was 19.2 mU/ml with 95% range 8-65 mU/ml for cyanotic (SaO2 less than 94%) and 17.7 mU/ml with 95% range of 12-36 mU/ml for acyanotic patients. In this group siEPO values were inversely correlated to the arterial oxygen content (log siEPO (mU/ml) = 1.61-2.04 x oxygen content (l/l); r = -0.28; P less than 0.02).