Haemophilia is a rare inherited disease of blood clotting known since biblical times. The rarer form (haemophilia B) occurs in about 1 in 30,000 males and there are about 900 patients in the U.K. at present. Biochemically, patients either lack or have a defective protein (called factor IX) which is needed for the clotting of blood in response to injury. Only males get the disease. However, females can carry the trait in a latent form and transmit the disease to their offspring. Untreated, the disease leads to internal bleeding into muscles and joints and is life-threatening. In the U.K. and in countries with effective health care programmes, patients are treated by periodic injection of factor IX concentrate, a drug isolated from the pooled plasma derived from many blood donors. This drug replaces their own absent or defective factor IX and allow them to enjoy a relatively normal lifestyle. I have reviewed recent studies on the molecular genetics of haemophilia B which started with the isolation of the gene coding the factor IX protein from normal individuals in 1984. Following this, it has been possible firstly to produce factor IX artificially in the laboratory from cloned copies of the messenger RNA of the factor IX gene. Secondly, it has been possible to improve the diagnosis of 'carriers'. Carrier females often wish to know whether they are carriers or not before they have children. If they are positively identified as carriers, the risk and implications of having a haemophiliac son can be discussed and therapeutic abortion considered.(ABSTRACT TRUNCATED AT 250 WORDS)