The various etiologies of spontaneous hemarthrosis in adolescents and adults are reviewed: they include systemic diseases and local or regional disorders of the bones or joints. Among systemic diseases, the two main causes are coagulation disorders and hemoglobinopathies. Coagulation disorders may be either acquired (leukemia, thrombopenia, and hypoprothrombinemia induced by anticoagulant drugs with hemarthrosis being one of the major complications) or inherited (hemophilia which is not considered here, von Willebrand disease, and congenital thrombopathies). Hemoglobinopathies, particularly sickle-cell disease, are responsible for hemarthrosis in a few patients. Among local or regional disorders of the bones or joints, tumors such as hemangioma or synovial sarcoma are uncommon causes. Hemarthrosis is the main feature of pigmented villonodular synovitis. Hemarthrosis may occur in degenerative and metabolic diseases: while it is extremely rare in arthritis, it is frequently encountered in articular chondrocalcinosis which is the first diagnosis to consider when hemarthrosis occurs in an elderly patient. The search for an etiology, which is often difficult, should include a review of prior illnesses, a study of coagulation, and local clinical, radiological and biological investigations, with a study of the synovial fluid; in some instances, arthroscopy, synovial biopsy and even surgical exploration are required. Management includes rest, analgesics, antiinflammatory drugs and, above all, arthrocentesis which is essential for the prevention of articular damage and functional sequellae. Specific therapy is dependent on the etiology. In recurrent hemarthrosis, isotopic synoviorthesis may ensure lasting resolution of the effusion.