In a series of 635 single non-malformed newborn infants with a gestational age ranging from 35 to 43 weeks we found venous haematocrit values of 60% or greater (range 60-72%) in 117 (18%); 30 neonates (4.7%) had haematocrits above 65%, and of these, 7 had values above 70%. Only 13 developed symptoms, all minor, which might be ascribed to polycythaemia, and all 117 survived. Haemodilution was not performed in any case. Ninety-eight children were followed up at the age of 2.5 years with a physical examination and a Denver Developmental Screening Test. At the age of 6 years an interval story concerning development and health was obtained from a questionnaire in 92 children; 4 children were lost to both follow-up investigations. Of the 113 children, 104 (92%) were classified as normal. The remaining children suffered from febrile convulsions (one case), nocturnal enuresis (five cases), speech defect (one case), gross motor clumsiness (two cases) and hyperkinetic behaviour (one case). None had epilepsy or cerebral palsy. The findings were minor and showed no association to haematocrit levels or possible polycythaemic symptoms in the neonatal period. Hence we do not consider preventive haemodilution to be indicated in neonatal polycythaemia with minor or no symptoms within the studied range.