The work for this thesis consists of both clinical and laboratory studies into the haemorrhagic problems of patients with haemophilia and Christmas disease. In diseases with so many facets it is not possible to examine every aspect so I have focussed my attention on some outstanding clinical and laboratory problems. In the clinical section I have investigated methods of measuring the early pathological changes in the synovium of the joints, the biochemical methods of diagnosing muscle bleeding, the localisation of muscle haematomata with ultrasonography, the effect of fibrinolytic inhibitors in the reduction of bleeding after dental extraction, the incidence of and aetiology of gastro-intestinal bleeding and evaluated electroencephalography as a test of previous cerebral bleeding. In the laboratory section I look at some of the complications of therapy with plasma products, both of human and animal origin. In particular the incidence of transfusion hepatitis, the incidence of blood group antibodies, the effect of elevation of plasma fibrinogen and the effects of animal antihaemophilic factor on human blood platelets.