18 thalassaemic children, aged 3.5 to 13 years comprise our clinical material. In 14 of them clinically elicited spleen markings, haematocrit, blood platelet count and red cell morphology were studied daily for the whole period between transfusions. In 10 patients considerable changes in spleen size were noticed. According to out clinical observations the spleen size starts decreasing 1 to 3 d after blood transfusion up to the 10th posttransfusion day fluctuating thereafter to reach its maximum size again prior to the next blood transfusion. The decrease of spleen size was followed by an increase of haematocrit and blood platelet count and vice versa. 4 additional children were studied clinically only twice: prior to and 7 to 10 d after blood transfusion. A definite decrease of the spleen size following blood transfusion was observed. Spleen and liver 99mTc-sulfur colloid uptake was studied in 10 of the above children prior to and 7 to 10 d after blood transfusion. Statistically significant post-transfusion increase of the spleen uptake was demonstrated. Our findings suggest that (a) splenic size is relevant to blood volume sequestrated int this organ, (b) splenic radioactive uptake increases with its post-transfusion reduction in size.