[Leprosy tests: diagnostic problems (apropos of 2 cases)]

Two cases of reactional leprosy leading to wrong diagnosis are reported. The first one concerns a reversal reaction predominantly neuritic, initially taken for polyarthritis. The second one concerns an erythema nodosum leprosum with extensive cutaneous necrosis (Lucio's phenomenon or ulcerative lazarine leprosy). Main aspects and mechanisms of leprosy reactional states are reviewed. It is emphasized that errors or delays in diagnosis are often caused by failing to recognize cutaneous or neuritic symptoms.