Twelve dogs suffering from acute lymphoblastic leukaemia were investigated concerning the following tests: platelet count, prothrombin time (PT, standard test, modified test), activated partial thromboplastin time (APTT), activity of the individual coagulation factors II, V, VII, X, VIII:C, IX, XI, XII, prekallikrein, and high-molecular weight kininogen, the activity of antithrombin III (AT III), protein C, plasminogen, and alpha 2-antiplasmin as well as concentration of fibrinogen, soluble fibrin and fibrin(ogen) degradation products (FDP). All patients showed a decreased platelet count due to suppression of megakaryopoesis by infiltration of the bone marrow with leukaemic cells. In addition, in most of the patients a moderate activity decrease of one or more individual coagulation factors has been found, especially regarding factor II (median, x0.50 = 51%, p = 0.0001), but also factors X (x0.50 = 71%, p = 0.0003) and XI (x0.50 = 68%, p = 0.0006). This was reflected by the APTT and the PT activity (modified test), which were prolonged or decreased, respectively, in the majority of the cases. Furthermore, the activity of AT III and of plasminogen was distinctly diminished (p < 0.001). Like the concentration of FDP, the plasma level of soluble fibrin was significantly higher than in normal dogs (p < 0.001). This indicates that besides thrombocytopenia disseminated intravascular coagulation occurs frequently in dogs with acute lymphoblastic leukaemia and is a main cause for the decreased activity of several plasmatic components of the haemostatic system. The lack of correlation between the concentration of soluble fibrin as an indicator of intravascular coagulation and the total blast cell count (rS = 0.011) shows the importance of other factors like degree of cell lysis as well as participation of organs such as the liver for generation of consumption coagulopathy in dogs with acute lymphoblastic leukaemia.