We report the appearance of a factor V inhibitor in an 82-year-old female patient following abdominal surgery. This anomaly was totally corrected following 9-day treatment with i.v. immunoglobulins (0.4 g/kg/day), which allowed a needed further surgical intervention to be performed without hemorrhagic diathesis. The activity of this inhibitor was partly reversed by the in vitro addition of immunoglobulins, suggesting a concentration-effect relationship. This finding supports the hypothesis that anti-idiotypic antibodies could play a role in the therapeutic effect of immunoglobulins in such situations.