In a study, gallbladder lesions due to Opisthorchis viverrini infection were controlled roentgenographically after treatment with praziquantel (2-cyclohexylcarbonyl-1,2,3,6,7,11b-hexahydro-4H-pyrazino[2,1-a] isoquinolin- 4-one, EMBAY 8440, Biltricide). The place of study was the Khon Kaen Hospital, Northeast Thailand. Included in the study were 20 patients with impairment of gallbladder function; 7 of them were males and 13 females. Their age ranged from 27 to 60 years (mean 47.4), eggs per gram (EPG) ranged from 800 to 30,000 (geometric mean 4321). The laboratory parameters were within normal limits. Praziquantel 3 X 25 mg/kg per kg body weight was given on Day 0 and all patients were parasitologically cured within 60 days. Oral cholecystography was performed to evaluate the function and size of the gallbladder and intravenous cholangiography for the common bile duct on Days 0, 14, 60, 120 and 180. The results were: (a) very large and severe dysfunction of gallbladder seen in three patients returned to normal in 60, 60 and 180 days; (b) moderate dysfunction was present in 13 patients, all but one returned to normal: 6 in 14 days, 1 in 21 days, 2 in 60 days, 1 in 120 days and 2 in 180 days. The only patient who showed no change had diabetes mellitus and moderate dysfunction of the gallbladder for one year prior to treatment; (c) mild dysfunction: all 4 patients were normal in 14 days. The common bile duct was enlarged in 3 patients (1.2, 1.3, 1.5 cm); no significant changes were seen.(ABSTRACT TRUNCATED AT 250 WORDS)