The contribution of the arc 5 double diffusion test (DD5) to the diagnosis of hydatidosis in two patients with previous surgery for this parasitic infection is described. In one of the patients, who was asymptomatic, the diagnosis was established on the basis of a positive DD5 reaction five years after her last surgical intervention. The abdominal location of cysts was revealed by computerized axial tomography (CAT). The other patient had been operated on for hydatidosis nine years earlier and showed symptoms. Positivity to the DD5 test confirmed the etiology of the disease, which had been suspected on the basis of her clinical history and the findings of scintillographic and CAT studies. In both cases, however, other abdominal cysts were found at surgery whose images had not been revealed in the previous studies.