Adverse events (AEs) associated with bisphosphonates are usually gastrointestinal, but elevation of aminotransferases (alanine and aspartate aminotransferase) has also been described in many cases. The frequency of such elevation is, however, poorly investigated. The Probone study is a continuing phase II trial of 610 osteopenic women randomized for 3 years to receive placebo or clodronate at the following clodronate doses: 65 mg, 400 mg and 800 mg daily, and 400 mg for 15 days out of 90 (intermittent group). During the first study year, gastrointestinal AEs were reported by 20-26% of the patients in the five study groups, but no differences appeared between groups. The high-dose clodronate groups (400 mg and 800 mg daily) had no more hepatic side-effects than the low-dose groups, and no serious AE due to liver disease was reported. These higher doses of clodronate caused, however, a mean alanine aminotransferase (ALT) elevation of 5.4-5.8 U (25-24%) which differed significantly from the change in the placebo group (p = 0.002 and p<0.0001, respectively). In those volunteers with initially normal aminotransferase values the risk ratio for an aminotransferase increase to an above-normal value (compared with the placebo group) during the study year was up to 1.8 in the clodronate 400 mg group and up to 2.5 in the clodronate 800 mg group. Among these groups an elevation above the normal limits occurred in up to 11.7% of the volunteers for aspartate aminotransferase (AST) and up to 17.7% for ALT. The respective percentages in the placebo group were 6.2% and 7. 2%. All analyzed bilirubin values were normal. We conclude that oral clodronate use may elevate serum aminotransferase levels, and that these enzymes most likely come from the liver. This elevation has also been described for several other bisphosphonates. However, clinically significant liver injury is unlikely.