Mebendazole and albendazole are the drugs of choice for the treatment of alveolar echinococcosis. In this open-labelled observational study we present and evaluate the outcome of long-term treatment with these drugs and present results of different treatment regimens. Thirty-five patients were started on either mebendazole or albendazole at the beginning of 1992 and followed for an average of 39 months (range 12-79 months). Treatment was classed as successful if the disease had not progressed for >1 year and if there were no side-effects necessitating a change of treatment. Lack of progression was evaluated mainly using ultrasound and computed tomography and was further substantiated by laboratory tests and clinical findings. The overall success rate was 97%. An initial regimen for cases of alveolar echinococcosis was recurrence-free in 71% of those treated with mebendazole and in 78% of those treated with albendazole. Four out of five cases with progressive disease stabilized after the therapeutic regimen was changed. Seven patients received a continuous regimen with albendazole. These patients were observed over an average of 28 months (range 13-50 months) without signs of progression or significant side-effects. This open-labelled prospective study demonstrates the high therapeutic efficacy of both mebendazole and albendazole with similar response rates in the treatment of alveolar echinococcosis. Albendazole reduced costs by >40% and is easier for patients to take, further arguing in favour of its preferred use. Albendazole in alveolar echinococcosis is only licensed for intermittent application. None the less, continuous treatment is safe and well tolerated and showed promising results when applied to patients in whom other treatment regimens had failed. It should thus be strongly considered in inoperable cases or progressive disease.