The results of total knee replacement in five patients aged between 22 and 37 with severe haemophilia A or B are described. All patients had been managed conservatively without success. Frequent bleeds, severe pain and limitation of movement were the indications for operation. Despite close haematological surveillance, bleeding problems occurred in three of the patients and large quantities of plasma concentrates were required. Review of the patients over a period of 25 to 48 months after operation showed dramatic lessening of pain and maintenance of a satisfactory range of movement. The frequency of haemarthrosis diminished markedly and the requirements for factor concentrate in the years after operation fell substantially. Two patients returned to employment. Total knee replacement led to marked clinical improvement in all the patients, but the long-term results are not yet known.