In two children with acute lymphocytic leukaemia in whom severe anaemia developed, serum samples collected over 9-12 months showed high concentrations of B19 parvovirus, the aertiological agent of fifth disease. In one patient, anaemia recurred with the reappearance of virus in serum; in the other, the anaemia persisted. Smears of bone marrow aspirates obtained during periods of viraemia showed giant pronormoblasts and either absent mature erythroid cells or erythroid hypoplasia. Parvovirus replication in the marrow was detected by Southern blot of cellular DNA. An underlying immune deficit was suggested by low titres of specific antibodies against B19 parvovirus. Treatment of one patient with plasma containing specific antibodies against parvovirus resulted in a transient fall in serum virus levels, the appearance of reticulocytes, and symptoms of fifth disease.