We have shown that activated partial thromboplastin time values in children are considerably longer than in adults, but the causes for this observation remained unclear. Therefore, we investigated the correlation between activated partial thromboplastin time values and concentrations of clotting factors, quotients and titers of the tissue thromboplastin inhibition test, and antiphospholipid antibodies in healthy children, children with recurrent infections, and adults. Concentrations of factors VIII, IX, and HMWK were significantly lower in children than in adults. Simple linear regression analysis failed to show a correlation between the concentration of a single clotting factor and the activated partial thromboplastin time values. No significant correlation was found between activated partial thromboplastin time and elevation of the tissue thromboplastin inhibition test quotients or titers, or antiphospholipid antibodies values. The determined activated partial thromboplastin time was best described by a function including all measured coagulation factors. Our study suggests, that no single clotting factor or lupus anticoagulants are responsible for the longer activated partial thromboplastin time in healthy children, but that activated partial thromboplastin prolongation is caused by the combination of several slightly lower clotting factors.