Recently, in an epidemiological investigation involving 1,218 children aged 11-14, we demonstrated that the prevalence of von Willebrand's disease, based on a low ristocetin cofactor activity (RiCof) in children with a personal and/or family history of hemorrhage, was at least 1% (Blood 1987; 69: 454). All the diagnosed cases had multimeric patterns typical of type I von Willebrand's disease (vWd). Since standardization of RiCof is difficult and the test is not easily performed in a clinical laboratory, we measured von Willebrand factor antigen (vWf:Ag) in all available unthawed plasma samples of previously investigated children by ELISA, to assess the relative sensitivity of this more simple test for diagnosing vWd. Separate normal ranges were calculated by non-parametric methods for 0 and non-0 subjects, and for children and adults, since values were higher in non-0 subjects and in children. Taking into account the 90% confidence interval around the lower limit of the normal range, 7 (50%) of the 14 cases diagnosed by RiCof were detected by vWf:Ag. Furthermore, two new cases would have been diagnosed by vWf:Ag, leading to a relative Ag/RiCof global sensitivity of 64%. A similar figure was obtained when the two tests were compared in the group of relatives of the affected children. In conclusion, measurement of vWf:Ag seems to be definitely less sensitive than the RiCof assay for detecting patients with vWd, even in type I patients, and RiCof remains the test of choice for screening for vWd in hemorrhagic patients.