To document the natural history of Echinococcus granulosus infection and response to treatment of human hydatidosis, we reexamined 28 of 37 subjects with E. granulosus infection diagnosed in an epidemiological study conducted in 1994. Twenty-six (70%) of those 37 subjects underwent abdominal ultrasonography, chest radiography, and enzyme-linked immunoelectrotransfer blot assay in 1997. Medical records from two additional individuals were reviewed. Eight patients had their cysts surgically removed during the 3-year follow-up interval; no surgical complications or recurrences occurred. Among eight patients with cystic disease not treated by surgery, four had cyst- growth ranging from 0.4 to 1.4 cm during the 3-year interval. One patient developed a new cyst and another's simple cyst became septate; two developed new calcifications. Of 12 seropositive subjects with no cysts present in 1994, 10 reverted to seronegative, a finding that suggests a significant proportion of seropositive subjects in echinococcus-endemic regions may have only transient infection without disease. When cysts do develop, their growth rates and time courses are highly variable; over the 3-year period, we observed growth, septation, degeneration, and calcification of cysts.