Hydatid disease is one of the most serious parasitic infection in humans and ruminants. In spite of this, no research had been conducted about the prevalence of this infection in the city of Arak, which is one of the centers for raising livestocks. In order to do this, medical files of 73 subjects, whom were diagnosed as having hydatid cyst ( between years 1983-1993) and had gone under operation in Qods hospital were studied carefully. A total of 4314 sheep, 5833 goats, and 1498 cattle, which were killed in the slaughter house of the city of Arak were also examined. The results indicated that, the highest percentage of infection was in housewives. The prevalence of infection was highest among patients who were between 40 to 49 years old ( male 45.2℅ , female 54.8℅). Liver was the most (78.8℅) and spleen was the least (1.4℅) affected organs, the studies showed the prevalence of hydatid infection in cattle (2.7℅), sheep (2.4℅) and goats (0.6℅), of these 92.5℅ of cysts in sheep. 82.3℅ in goats , and 5.8℅ in cattle were active cysts containing scolex.