Prothrombin (Factor II, FII) deficiency is an extremely rare autosomal recessive condition with an estimated incidence of 1:2 million. As severe and life-threatening bleeding is rare in FII deficiency, on demand therapy with administration of prothrombin complex concentrates (PCCs) or fresh frozen plasma is generally performed, and prophylactic therapy for FII deficiency has been reported in only three cases. Thus, its optimal dosage and schedule has remained uncertain. Here we report a case of severe prothrombin deficiency with a novel frameshift mutation of the F2 gene, who was started on prophylactic administration. Pediatr Blood Cancer 2013; 60: 503–505. © 2012 Wiley Periodicals, Inc.