Antiphospholipid syndrome is a systemic disorder characterized by arterial and/or venous thrombosis, thrombocytopenia, recurrent fetal loss, and presence of antiphospholipid antibodies (APA). The importance of APA in Turkish children with thrombosis is unknown. This study aimed to evaluate the frequency of APA positivity, associated risk factors other than APA, and outcome in children with APA and thrombosis. The presence of APA was investigated in 138 children presenting for evaluation of thrombosis; other prothrombotic risk factors were also studied. The frequency of APA positivity among 138 children was 11.6% (16/138). The mean age of these 16 children (10 female, 62.5%) was 9.57 ± 4.59 years (range, 2.5-18.0 years). The mean follow-up period was 31.7 ± 21.7 months (range, 5-60 months). Recurrence was observed during follow-up in two patients (12.5%). Ten patients (62.5%) had arterial thrombosis, five patients (31.3%) venous thrombosis, and one patient (6.3%) purpura fulminans. Among the thrombotic children with APA, 11 (68.8%) had more than one prothrombotic risk factor other than circulating APA [five patients (31.3%) had two risk factors, two patients (12.5%) had three, and four patients (25.0%) had four]. Five patients (31.3%) had no additional risk factors. APA should be tested in all children with thrombosis, especially those with arterial thrombosis. © 2007 Lippincott Williams & Wilkins, Inc.