A newborn infant was diagnosed as having congenital basophilic leukemia, an extremely rare disorder, by examination of cord blood. An ultrastructural cytochemical study was conducted on the blasts of the infant. Granules of basophil blasts showed ultrastructural characteristics of immature basophil granules and were similar in size to those of normal basophil granules but were fewer in number in each profile. Histochemical studies showed weak reactions for acid mucopolysaccharides and neutral glycoconjugates in the granules, but they were negative for peroxidase. Acid phosphatase was markedly positive. The findings of congenital basophilic leukemia are different from those of basophils in chronic myelocytic leukemia. The patient underwent induction therapy with vincristine and prednisone and has been in complete remission for 21 months.