Seven patients are described who suffered from the aplastic anaemia-paroxysmal nocturnal haemoglobinuria syndrome. All had been first diagnosed as having aplastic anaemia. In one case the bone marrow became repopulated but the patient developed severe intravascular haemolysis and died of liver failure due to intrahepatic venous thrombosis. Two patients died of marrow aplasia; haemolysis was minimal and was only a laboratory phenomenon. Another patient also died of marrow aplasia but in his case haemolysis had contributed significantly to his illness. In three patients the bone marrow became repopulated completely or partially and the patients have recovered clinically despite the persistence of a small proportion of PNH red cells.

The relationship between aplastic anaemia and PNH is discussed. It seems likely that the link between the diseases is the development of an abnormal clone of haemopoietic cells in a regenerating, previously aplastic, marrow.

The effect of the superimposition of the PNH defect on the prognosis in aplastic anaemia is discussed. In five cases haemolysis had little or no influence on the course of the disease; but in two patients intravascular haemolysis was severe and was undoubtedly harmful.

The influence of splenectomy on the course of the disease was studied. It was performed in four of the patients, but in only one did it seem to have a beneficial effect.