The authors describe the evaluation and course of severe hereditary prothrombin deficiency in a 14-year-old girl first diagnosed at age 4 years. Detailed is the evolution of her treatment from episodic fresh-frozen plasma after bleeding events to prophylactic home infusions with the prothrombin complex concentrate Bebulin VH. Pharmacokinetic data on factor II recovery and half-life are presented. The patient has been essentially free of abnormal bleeding while on this prophylactic regimen for 17 months, with no toxicities and with a much improved quality of life.