The prevalence of antiphospholipid antibodies (APAs) has not yet been studied in children and adolescents with various diseases. We assayed plasma samples of 203 children and adolescents, aged 0.1 to 21 years (median 6 years), by enzyme-linked immunosorbent assay detecting immunoglobulin G (IgG) and IgM antibodies directed against cardiolipin, phosphatidylserine, and phosphatidic acid. According to their diagnosis children were divided into five groups: 10 patients with autoimmune and autoimmune-like diseases (group I); 88 patients with infections (group II); 20 patients with metabolic diseases (group III); 65 children with various other diseases (group IV); 20 healthy children without physical illnes (group V). Results were compared for statistical significance using Fisher's exact test. APAs were found in 65 of 203 patients. In all groups of patients the prevalence of APAs was increased compared with healthy children. The highest prevalence of APAs was found in patients with autoimmune or autoimmune-like diseases or infections. Thrombosis did not occur in children with increased APA levels. In 4 of 65 patients with detectable APAs bleeding was observed. However, three of these four children had additional coagulation abnormalities. We conclude that in children and adolescents increased APA titers may occur in a variety of diseases. The frequency of APA-related symptoms is low.