Ceftriaxone has potent activity against a broad range of Gram-positive and Gram-negative bacteria. While it is eliminated mainly by the kidney, 10-20% of the drug is eliminated in the bile and ceftriaxone salt precipitates have been described in the gallbladder of animals dosed with ceftriaxone. The purpose of the present study was to investigate the incidence of biliary lithiasis 6 and 12 months after treatment with ceftriaxone and to compare it with that in patients treated with amoxycillin/clavulanate. Biliary ultrasonography was performed at the start of treatment, at 6 months and at 12 months after the beginning of the study. One hundred patients were randomized and 74 were evaluable: 34 were given amoxycillin/clavulanate, 40 ceftriaxone. Gallbladder lithiasis developed in one patient 12 months after the amoxycillin/clavulanate treatment and in none in the ceftriaxone treatment arm. Biliary precipitate during ceftriaxone treatment was not looked for because this phenomenon was not known at the beginning of the study, but gallbladder precipitation that was seen in two patients given ceftriaxone during and at the end of treatment, respectively, resolved spontaneously. In conclusion, ceftriaxone treatment does not appear to lead to gallstone formation more often than an antibiotic that is not eliminated through the bile.