The authors report a case of hemarthrosis complicated by severe anemia related to a congenital connective tissue disease: Ehlers-Danlos syndrome. A boy fell down and suffered tumefaction of both knees with bilateral rupture of the rotula tendon. He underwent surgical reinsertion of each tendon on the rotula. He later showed an unexpected ongoing hematic effusion, with severe anemia. He was screened for coagulation disorders with no results. On taking a more detailed history and investigating the patient's phenotypical features, the authors diagnosed Ehlers-Danlos syndrome, hypermobile variant. The hemarthrosis and anemia were thus concluded to be consequences of excessive tissue fragility due to a congenital connective tissue disease.