The serum and cyst fluid levels of selenium, zinc and copper were investigated both in patients and sheep with hydatid cysts. Results were compared with those of healthy subjects in both species. The serum values of Se, Zn and Cu in healthy subjects were 60.24 ± 1.96 μg/L, 0.59 ± 0.07 mg/L, and 1.01 ± 0.08 mg/L, respectively. The corresponding serum values of the three trace elements in patients with hydatidosis were 41.30 ± 1.94 μg/L, 1.27 ± 0.08 mg/L, and 0.89 ± 0.09 mg/L. These results show a decrease in Se and Zn, and a rise in Cu in sera of patients with hydatid disease. Similar results were obtained in infested sheep as related to the healthy control animals. Our results on the three trace elements were found to correlate well with the incidence of cirrhosis, malignant and infectious diseases and arthritis. Se was not detectable in cyst fluid, while Cu and Zn levels were found significantly lower than serum levels. Cu and Zn levels in cyst fluid showed no difference in both species. Variations in serum Cu, Zn and Se levels were insignificant with regard to the genus of the host and the fertile capacity of the hydatid cyst.