The evaluation of microhaematuria may be considered as a measure of the haemostatic defect of haemophiliacs in nonhaemorrhagic periods. In order to study the relationship between coagulation disorder and haemostatic defect we have administered small amounts (1 U and 3 U/kg) of AHG and of factor IX (1 U/kg) to 12 haemophiliacs (10 A and 2 B) and we have examined the effects on coagulation and microhaematuria. Microhaematuria was determined by the measurement of urinary radioactivity/24 h, after infusion of autologous RBC labelled with 51Cr, for 11 consecutive days (4 days before and 7 after the administration). Coagulation was investigated by thromboelastography, PTT, Quick’s time, PCI, factor VIII and IX activity assays, fibrinogen assay, and euglobulin lysis time. During the period of investigation, the patients remained in bed. In controls, factor VIII administration did not affect microhaematuria. On the other hand, in all haemophiliacs, factor VIII and IX administration significantly reduced microhaematuria, although the administered dose was not able to modify the coagulation defect. In all subjects the reduction of microhaematuria lasted at least 5 days. The correction of the haemostatic defect, without improvement of the clotting disorder, and its persistence for several days suggest that factor VIII is involved in the composition of the vascular wall and in its functional integrity. Moreover, on the basis of these results it is possible to carry out a rational prophylaxis of the haemorrhagic episodes using small doses of factor VIII, by determining the duration of the correction of the haemostatic defect for each subject.