1. Ten studies were performed on nine patients with haematological disorders but with normal lungs, who required intermittent blood transfusions. The transfer factor for carbon monoxide and uptake of carbon monoxide per unit lung volume (KCO) were measured with the single breath technique before and at various intervals after transfusion. 2. The mean haemoglobin concentration increased from 7.7 to 11.1 g/dl. 3. The TLCO increased according to a formula based on the Roughton & Forster (1957) diffusion equations. TLCO (standardized) = TLCO (observed). (10.2 + Hb)/1.7 Hb, where haemoglobin (Hb) is expressed as g/dl. 4. The correlation between measured and predicted values was slightly better if changes in alveolar volume were taken into account, by using the KCO value.