Type IIb von Willebrand's disease has been found to be associated with the development of thrombocytopenia following the infusion of DDAVP (desmopressin). It has also been associated with sporadic thrombocytopenia and evidence of spontaneous platelet aggregation. A family with documented Type IIb von Willebrand's disease is described, where two of the affected females presented with moderate to severe thrombocytopenia developing during pregnancy with reversal to normal or minimally reduced platelet counts in the early post gestational period. In each case, the levels of factor VIII:C, von Willebrand factor antigen and von Willebrand factor ristocetin co-factor activity rose during pregnancy but there were notable discrepancies between the levels of each in any one individual. It is suggested that pregnancy resulted in increased synthesis of the variant form of von Willebrand factor resulting in progressively increasing platelet/variant form von Willebrand factor interaction and subsequent thrombocytopenia. Whether this reflects consumption or sequestration remains uncertain. Although spontaneous platelet aggregation was observed in some family members, the majority did not exhibit this phenomenon. Circulating platelet aggregates could not be detected. Both pregnancies were relatively uneventful and there is no history of unusual bleeding associated with pregnancy in the family. These observations suggest that Type IIb von Willebrand's disease should be considered in the differential diagnosis of thrombocytopenia developing during pregnancy, particularly in those individuals where evidence supporting the diagnosis of immune mediated thrombocytopenia is not forthcoming. Where the diagnosis of Type IIb von Willebrand's disease is established, active intervention other than confinement in a hospital with experience in haemostatic disorders is probably not required as the development of thrombocytopenia does not appear to exert an additive effect on the underlying defect relating to the variant form of von Willebrand's disease.