AbstractThe occurrence and causes of a haemodilution anaemia have been studied in two groups of patients, one with diseases of the blood or reticulo‐endothelial system, the other with cirrhosis.In the blood dyscrasia group 37 of 46 patients were anaemic but only 3 had a reduced red cell mass. In the remainder the plasma volume was increased to a greater extent than the redcell masswhich was slightly elevated or within the normal range. A significant correlation was found between a decrease in haemoglobin concentration and an increase in plasma volume. Both the increase in plasma volume and the changes in red cell mass were related to enlargement of the spleen. Measurements carried out at splenectomy showed that an excess of red cells was held within the enlarged spleen whereas up to 30 per cent only of the excess plasma could be found there. A correlation between the plasma volume and spleen blood flow was found but both are related to spleen size and the most likely explanation for the increased plasma volume was a primary increase in vascular capacity. The different effect of posture on the plasma volume in patients before and after splenectomy would also support this hypothesis.In contrast, 20 of the 50 cirrhotic patients were anaemic and seven of these had a reduced red cell mass. In the other 13 patients the plasma volume was increased to a greater extent than the red cell mass but even in those with a haemodilution anaemia, the haemoglobin concentration was not inversely related to the plasma volume. In seven patients the increase in plasma volume was less than that of the red cell mass and four patients were polycythaemic. There was no correlation between the changes in red cell mass and plasma volume or between either of these and the enlargement of the spleen. The presence of portal hypertension appeared to be an important factor but the increase in plasma volume was not related to the extent of the portosystemic venous collateral