Type III von Willebrand's disease (vWD) was diagnosed in 38 Dutch kooiker dogs. Ten male and 9 female probands had been referred independently of each other to the Utrecht University Clinic for Companion Animals because of a moderate to severe bleeding tendency. Screening of 717 Dutch kooiker dogs, including 356 puppies, detected vWD in another 19 dogs. Diagnosis was based on non-detectable amounts (< 1.6%) of von Willebrand factor antigen (vWF:Ag) in plasma by ELISA. Capillary bleeding time (CBT) was prolonged (> 10 min) and polybrene cofactor activity (vWF:PbCo) was not detectable in 11 dogs tested. No distinguishable protein bands were detected by multimer analysis. As in Scottish terriers with type III vWD, factor VIII clotting activity (FVIII:C) in affected Dutch kooiker dogs was decreased but considerably less than in humans with type III vWD. A recessive mode of inheritance was indicated by the normal or subnormal but measurable amounts of vWF:Ag in the plasma of eight pairs of parents of affected dogs. The F1 offspring resulting from the experimental mating of two affected dogs consisted of three affected males and four affected females. In 39 obligatory carriers vWF:Ag ranged from 30% to 114% with median and mean vWF values of 64% and 64.2%, respectively, and was subnormal (< 50%) in only 9 animals.