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A 69-year-old man of Jewish descent with a second local relapse of rectal carcinoma was found to have a markedly prolonged activated partial thromboplastin time (aPTT). Further evaluation revealed a homozygous factor XI deficiency. Despite various operations in the past he had never displayed any bleeding problems. Severe factor XI deficiency did not prevent venous thrombosis in this patient. The management of patients with prolonged aPTT is described and insight into the pathophysiology of factor XI deficiency is provided. The differential diagnosis in patients with a prolonged aPTT depends on their bleeding tendency. There is a large variability in bleeding tendency in patients with factor XI deficiency. Patients with factor XI deficiency and an increased bleeding tendency can be treated with antifibrinolytic agents prior to small interventions, such as tooth extraction, or with plasma prior to surgery.