Abstract: The study of hemostasis often arises in paediatrics. Evidence of activated partial thromboplastin time (aPTT) prolongation sometime due to the presence of a circulating anticoagulant (antiphospholipid syndrome [APS]) may be embarrassing for the physician. Aim of the study: To evaluate the prevalence of this situation, to identify the leading indicators and assess their impact. Patients and method: All children aged 1 to 18 years old undergoing blood sample whatever was the reason, at the Nice University Hospital with existing isolated aPTT prolongation, were included. The assessment was completed by a mixing test, calculation of Rosner''s index as well as the study of an APS and the measurement of factor VIII, IX, XI, XII. Results: Between July 2006 and March 2008, 27 of 1845 children observed (1.5%) were selected for further study. Mean age was 6.17 years old. For 16 of the patients, aPTT prolongation was fortuitously discovered. Symptomatic subjects were older (9.8 vs. 5.2 years of age; P =0.03). A significantly higher aPTT was indicative of an APS and predicted a positive Rosner Test outcome. A prolongated kaolin clotting time, observed among the younger subjects (3.45 vs. 8.88 years of age; P =0.0011), was associated with a high aPTT prolongation (57.3 vs. 42.6s; P =0.0009). Conclusion: In our study, the discovery of a prolongated aPTT is most often incidental and tends to occur during winter. The presence of a highly prolongated aPTT, abnormal kaolin clotting time and positive Rosner Test are strong predictors of the existence of an APS, especially in very young children. These antibodies are nonpathogenic and transitional. [Copyright &y& Elsevier]