Eleven cases of alveolar echinococcosis (Echinococcus multiZocuZaris infection) with non-resectable lesions but treated with albendazole for 17 to 69 months were followed-up clinically and serologically for 45-l 1.5 years. Based on the clinical outcome and computerized tomography (CT) scanning, they were divided into 4 groups of 2 cured cases, 5 stabilized cases, 3 cases with recurrences, and one treatment failure. Forty-seven sequentially collected sera from the 11 cases were analysed by sequential enzyme-linked immunosorbent assay (ELBA) using Ern2PIUS antigen (Em2plus-EuSATM) andwestern blotting to detect antibody response against Em 18 (Em 18-Western blots). The antibody levels in one of the cured and 2 of the stabilized cases fell below the cut-off level in the Em2plus-ELISA TM 45-6 years after effective treatment, whereas all other cases, including 2 of those with recurrences, showed large reductions initially but increased again during the follow-up period. Em 18-Western blots of the 2 cured cases and 2 of the stabilized cases became negative. IgG subclasses with responses against Em18 which fell to zero included IgGl (2), IgG3 (one) and IgG4 (one). All other cases showed no decrease in antibody response against Em 18. There were, in general, reasonably reliable correlations between the success or failure of chemotherapy and antibody responses by Em2plus-ELISATM and Eml8-Western blots. These results suggest that both Em2plUS-ELISA~~ and Em18-Western blot are potentially useful in evaluating and predicting the efficacy of chemotherapy.