Fifty-eight patients with surgically confirmed hydatid disease and 38 age and sex matched healthy controls were included in this study. The demographic distribution of patients was recorded. A Blood sample was collected from each individual. Cytogenetic analyses of peripheral blood lymphocytes of each individual were performed which included the mitotic index (Ml), the cell cycle progression (CCP), replicative index (Rl) and sister chromatid exchange (SCE). The serum level of adenosine deaminase was evaluated for each individual. Indirect hemagglutination test (IHA) for hydatid disease was carried out for each individual. The samples from healthy controls that gave positive IHA results were excluded from this study. The incidence of hydatid disease in females was significantly higher than the males. This may be due to the fact that females are confined to housework, dealing with contaminated food with the eggs of the parasite especially in rural areas where the intermediate and definitive hosts (livestock and dogs) are found. The highest age distribution of patients with hydatid disease was between 20-30 years of age. That could be due to the chronic nature of the disease. The majority of patients were from rural areas having a higher chance of acquiring the disease because of the availability of the source of infection (the dogs), and the low hygienic level in such areas. The results of IHA on patients sera revealed that six of them had negative results. This could be expected for five of them, because the site of involvement is the lung, with its known poor humoral immune response in such cases. The sixth patient had cysts in the liver and lung according to the sonography report. The hydatid cyst of the lung was surgically proved, whereas the hydatid cyst of the liver was only reported by sonography. Sonography could not necessarily confirm the presence of hydatid cyst unless surgery is done. The mean MI of patients with hydatid disease was significantly lower than that of the healthy controls. The mean frequency of SCE in those patients was significantly higher than that of the healthy controls. The mean RI of those patients was significantly lower than that of the healthy controls. Most of the cells were inhibited at the first metaphase (M1) stage and only a few percentage of cells reached the second metaphase (M2) stage, while less than 8% of the cells reached the third metaphase (M3) in the patients. The cytogenetic results could indicate that the parasite may have a cytotoxic effect on lymphocytes. There was a significant decrease in serum adenosine deaminase (ADA) levels in patients with hydatid disease when compared with its level in the healthy controls. The reduction of ADA level could be related to the immune unresponsiveness state associated with hydatid disease.