Reports on variants of von Willebrand's disease are numerous, but many of these are based on tests that will show marked fluctuations with time and tests that might not be similar in affected family members. This report describes 8 patients with a new variant of von Willebrand';s disease in which there is a normal APTT, slightly reduced one-stage factor VIII:C assay (VIII:C-1), and a drastically reduced two-stage factor VIII:C assay (VIII:C-2). The VIII:C in this variant is more readily adsorbed to AI(OH)3. This variability in VIII:C assays and excessive adsorption to AI(OH)3 are corrected by the addition of either hemophilic plasma or hemophilic factor-VIII-related antigen. This variant is stable with restudy on multiple occasions and is inherited in a stable fashion in three generations of one family. The multimeric structure of the VIIIR:Ag appears normal, although the concentration is moderately reduced. The differences in functional activity, the adsorption to AI(OH)3, and the differences between functional and antigenic (VIII:C Ag) assays of VIII:C support that this is a functional abnormality of type I von Willebrand's disease.