The management of polycythaemia with hyperviscosity in newborn babies who are clinically well, or who only have minor signs, is not clear. Forty-nine such babies were randomly divided so that 24 were given a partial plasma exchange transfusion and the others were left hyperviscous. The babies were compared with normal controls. Clinical signs were more frequent in exchanged babies, and one developed necrotizing entercolitis. Of the hyperviscous babies 41% had plethoric lungs, 12% abnormal electrocardiograms, 9% were hypocalcaemic and 30% hypomagnesaemic. Behavioural testing after birth revealed differences in both groups when compared with controls. There were more poor scores in the exchange transfusion group. Neurological examination did not reveal marked differences among the groups. Developmental and neurological achievement at 8 mth of age was normal in all babies. In the present study it is suggested that newborn babies with hyperviscosity who are clinically well or who only have minor signs do not necessarily benefit from partial plasma exchange transfusion.