The main purpose of this article is to answer the questions about which test to perform for hydatic diagnosis and when. Several techniques for biologic diagnosis and follow-up of human cystic hydatidosis are reviewed. The specificity and sensitivity of immunologic reactions are reported. The differential diagnosis between Echinococcus granulosus and E. multilocularis is examined. The characteristics of the immunologic diagnosis according to the stage and the treatment of hydatidosis disease is discussed. Laboratory diagnosis of cystic hydatic disease is complementary to the clinical data. A judicious association of the usual techniques (indirect immunofluorescence assay, indirect hemagglutination assay, immunoelectrophoresis, co-electrophoresis with antigen 5 identification) confirms the diagnosis in 80% to 94% of hepatic hydatidosis cases and in 65% of pulmonary hydatidosis cases. Special techniques (enzyme-linked immunosorbent assay, Western blot, polymerase chain reaction) must be used for other localizations or when cysts are calcified. A serologic survey is necessary for the follow-up of operated medically treated patients. Despite poor standardization, purified antigens can distinguish between E. granulosus and E. multilocularis infections, although false-positive results are observed during other helminthiases, such as cysticerocosis.