Nineteen cases of leukocyte agglutination were observed in the last 6 years, while performing routine blood cell counts with an autoanalyzer (Technicon H6000). The phenomenon occurred only in edetic acid-anticoagulated samples and only at room temperature. Leukocyte aggregation could be reproduced by incubating patient serum samples or edetic acid-plasma with whole blood from ABO-compatible healthy control subjects. When patient serum samples or plasma samples were first preincubated with anti-IgM serum and then mixed with normal blood, leuko-agglutinates were not observed, suggesting that a plasma factor, probably an IgM, was responsible for the phenomenon. Nonetheless, IgM presence could not be correlated with any particular disease or the use of specific drugs.