Etiological analysis and parasitological examinations of human echinostomiasis were investigated. The four volunteers (case A, B, C and D) were orally administrated metacercariae harvested from the fresh water snails (Lymnaea japonica) experimentally infected with Echinostoma hortense eggs. Numbers of metacercariae administrated were 20 for cases A and B, 30 for case C and 50 for case D. Prepatent periods of four cases were 14 days (case D) and 17 days (cases A,B and C) after administrations. Patent periods of egg output were 18 days (case A), 32 days (case C), 207 days (case B) and 209 days (case D). The latter of two cases (B and D) showed the longest period of egg output in human echinostomiasis. Case A revealed the symptoms of violent watery diarrhoea and abdominal pain from 3rd to 4th weeks after administration, but others were asymptomatic. Maximal eosinophilia (22-35%) in peripheral blood of the three cases (cases A, C and D) was recognized 4 weeks after administration and the values decreased gradually thereafter. Case B did not show eosinophilia during the experimental period. Total serum IgE value measured by RIST method did not rise in all cases.