Background. Postoperative recurrence of the liver hydatid disease befalls approximately 10−30% of patients. Preoperative or postoperative therapy with albendazole in single therapeutic protocol (800 mg/d, within 28 days) indicated the need to evaluate the hydatid cyst liquid protoscoleces viability. Morphological changes of protoscoleces following the treatment with drugs are not well known. Aim. To estimate the viability of protoscoleces after preoperative or postoperative albendazole therapy, and their ability for cystic metamorphosis. Methods. A prospective, randomized clinical trial included 30 patients with liver hydatid disease, treated with albendazole and surgically (I group), and 30 patients in the control group treated only surgically (II group). The concentration of albendazole and its active metabolite albendazole sulphoxide in the cysts contents were determined using HPLC. Estimation of protoscoleces viability was based on the established micromorphologic criteria, and compared between the patients treated with albendazole, and the patients treated only surgically. Biological assessment of the viability was performed on protoscoleces with uncertain signs of the disturbed viability (unchanged structure, evaginated, without movements) using intraperitoneal injection of 1 ml of protoscoleces prepared suspension to AO type of rats. Results. The concentration of albendazole in cysts' contents ranged from 0 to 64.9 μg/ml, and of its active metabolite from 0.5 to 40.8 μg/ml. The presence of fully viabile protoscoleces in the albendazol treated patients was significantly lower than in the control group. A significant difference was noticed in the presence of disintegrated protoscoleces without movements in the albendazol treated group, compared to the control group. Biological assessment of the viability showed incapability of these protoscoleces for cystic metamorphoses. Conclusion. Low viability of parasites due to medicamentous therapy is very useful and important to surgeons, because the fertility of cysts is lower, and the risk of the disease recurrence is reduced.