In a review of the literature published in 1984 we identified (from 13 kindreds) 15 patients with severe von Willebrand's disease (vWD) reported to have developed alloantibodies. In order to reexamine the problem, a questionnaire was sent in 1989 to 141 hemophilia centers in America and Europe; 101 answers were received. These dealt with a total number of 1,720 patients with vWD (all types) and 231 patients with severe (type III) vWD. Besides the 15 cases reported in 1984, 6 additional cases of anti-vWF alloantibodies were reported, i.e., one from Spain (a relative of a previously reported case), two from Venezuela (brother and sister) and three from North Carolina (unrelated patients). All these cases and those previously reported share the following features: occurrence of alloantibodies in multitransfused patients with severe vWD, poor clinical and laboratory response to replacement therapy with cryoprecipitate, inhibition of ristocetin-induced platelet aggregation. During the survey, an unusual antibody to FVIII:C with no reactivity towards vWF was reported in a multitransfused girl (from Tennessee) with type III vWD. It seems therefore that although the development of alloantibodies is a rare complication of severe vWD, due to its negative effect on the response to treatment it needs a prompt identification.