The phenomenon of in vitro platelet clumping and consequent pseudothrombocytopaenia in the presence of EDTA has been studied in 33,623 subjects referring to a general hospital in a 1-year period. The observed frequency was 0.13%. EDTA-dependent pseudothrombocytopaenia (PTP) was suspected when a routine blood counting by the Coulter S-Plus IV/D showed a peculiar leucocyte histogram and pseudoleucocytosis. Confirmation was obtained by the manual count and by the finding of platelet aggregates in a stained blood smear. EDTA-dependent PTP was diagnosed when the platelet number and the morphological examination of blood anticoagulated with sodium citrate from the same patient were normal. EDTA-dependent PTP was found in 23 subjects aged from 19 to 79 years (0.068% of the study population): 17 were patients suffering from miscellaneous diseases, while six were apparently healthy. As a rule, platelet clumping was evident within 60 minutes from blood collection, but a longer latency (2-3 h) was observed in a few cases. EDTA-dependent PTP is a rare, but misleading phenomenon, the recognition of which is important in order to avoid expensive and potentially harmful procedures.