The practical organization of oral anticoagulant therapy with pediatric patients is often difficult. As an encouring alternative portable coagulation time monitors were developed in the last years to measure the prothrombin time with capillary blood. To evaluate the reliability of this method during childhood we examined in a prospective study 334 blood samples (175 capillary, 149 venous, 10 arterial blood) of 94 children (age: 1-18; median: 6,11 years) with and without phenprocoumon treatment with the CoaguChek-system, Roche diagnostics. The measurements were compared to a standard laboratory coagulation test (KC 10 / KC 40). In the group of children with phenprocoumon we found a linear correlation of the two methods (r = 0,96; y = 1,12x - 0,28). The measurements were not dependent on laboratory parameters like leucocyte or platelet count, hematocrit and bilirubin. The capillary, venous and arterial samples were comparable. Simultaneous treatment with heparin leads to overestimation of the prothrombin time (INR) by the CoaguChek system. Though we think with an appropiate, responsible regime of oral anticoagulation there is no further risk for thrombembolism. The measurements of CoaguChek on children without oral anticoagulation with normal prothrombin times in the range from 0,9 to 1,4 INR were inexact. Conclusion: The capillary prothrombin time (CoaguChek) is a sufficient method for children with phenprocoumon therapy and facilitates self-determination under medical control.