In a large-scale Bulgarian study, 122 patients with abdominal and/or lung echinococcosis were randomly selected for treatment with albendazole or mebendazole. The main aims were to evaluate the effect of each drug on the hydatid cysts, to follow the changes in cyst morphology during and after treatment, and to determine how quickly each drug produced the first, detectable, degenerative changes in the cysts. Follow-up was based on periodic ultrasonography, chest radiography and computed tomography. The abdominal cysts were categorised as small (<5 cm in diameter) or large. As albendazole treatment had effects that were almost identical to those of mebendazole treatment, the results for the two drugs were combined. In the lungs and, particularly, in the abdomen, the size of the cysts being treated influenced the character and timing of the degenerative changes seen in them. The initial change seen in each abdominal cyst was detachment of the endocyst, which occurred 1-3 months (small cysts) or 2-5 months (large cysts) after the initiation of treatment (P<0.05). The abdominal cysts then developed a hyper-echoic/hyper-dense appearance, became smaller, and finally disappeared 3.3-9.3 months (small cysts) or 5.6-13.9 months (large cysts) after treatment began (P<0.05). The first degenerative change noted in the lung cysts was cyst rupture, which occurred as early as day 10 of therapy but was generally observed 1 or 2 months after treatment began. After their complete evacuation, the ruptured lung cysts shrank and became deformed, some disappearing within 5-9 months of the initiation of treatment. The degenerative changes recorded, which began significantly earlier in the lung cysts than in abdominal cysts, indicate serious damage to the cysts and the parasiticidal, curative effect of each of the two benzimidazoles employed.