The haematoma in the sheath of musculus rectus abdominis is a rare clinical entity. Its diagnosis is often difficult. Authors observed it in three patients who were on anticoagulant therapy. All subjects were female; two of them had decreased renal function. All three patients received thrombocyte aggregation inhibitor therapy as well. The anticoagulant treatment (low molecular weight heparin in two cases, warfarin in one case) caused excessive anticoagulant effect. The diagnosis, which could be suspected after the physical examination, was established in two cases by ultrasonography and in one case by computed tomography. Two patients were surgically treated while the third patient was healed with conservative treatment. To prevent the rectus sheath haematoma, authors suggest a particularly careful control of anticoagulant treatment in older females and in subjects with decreased renal function, especially if the patient takes thrombocyte aggregation inhibitors, too.