Serial haematological investigations were carried out in 5 patients with Fanconi's anaemia over periods of 6 months--11 years. All the patients were pancytopenic with a depression of the granulocytic and megakaryocytic elements of the bone marrow throughout the greater part of their illnesses. Erythropoietic acitvity was variable. The initial bone marrow examination revealed depressed erythroid function in 3 patients. The erythroid hypoplasia persisted in 2 of them, while in the third, erythroid activity increased with time, possibly as the result of therapy with oxymetholone. Erythroid hyperplasia was present in the remaining 2 patients, both at presentation and throughout the course of the illness. This could not be attributed to treatment in either patient. Six erythrokinetic studies were carried out in the 5 patients at variable intervals after the diagnosis had been made. In 2 studies erythroid activity was unequivocally depressed, while in a further 3 a significant, though probably suboptimal, erythroid marrow response was present. In the final study erythropoiesis was increased but was markedly ineffective in terms of the delivery of viable red cells into the circulation. In vivo counting suggested that some degree of ineffective erythropoiesis was also present in the other patients and studies with 51Cr indicated a shortened red cell survival in all subjects studied. In 2 of them significant splenic sequestration was present. Leukaemic transformation occurred in 2 patients. In 1 of them its development was heralded by the appearance of micromegakaryocytes in the bone marrow.