Summary Background, objectives and methods: An accurate, sensitive and precise assay for reliable determination of the ristocetin cofactor activity of von Willebrand factor (VWF:RCo) in plasma and von Willebrand Factor (VWF)-containing concentrates has been evaluated. The assay is based on a commercially available automated protocol with modifications including a combination of adding additional ristocetin and the use of two calibration curves for the high and low measuring ranges. Results: Addition of extra ristocetin resulted in improved measurement of VWF recoveries from various VWF-containing concentrates that were underestimated using the standard automated protocol. The modifications resulted in improved assay performance over an extended measuring range (2.00 - 0.03 IU/mL). Accuracy was tested using VWF deficiency plasma spiked with 1(st) international standard for VWF concentrate. Seven dilutions, ranging between 1.80 and 0.05 IU/mL, were analyzed and resulted in measured concentrations between 80% and 100% of the assigned potency of the standard. Linearity was determined from the regression plot of the same concentrate dilutions and resulted in a correlation coefficient of 0.998. The repeatability, expressed as coefficient of variation, was 2% in the normal range (0.90 IU/mL) and 8% at the level of 0.05 IU/mL. The corresponding reproducibility results were 2% and 15% at the normal and low measuring ranges, respectively. Conclusions: Analysis of patients with von Willebrand disease indicates that the modified automated BCS((R)) protocol has a superior discrimination power compared with the standard protocol. This is especially true in samples with low VWF, as in patients with type 3 von Willebrand disease.