Ursodeoxycholic acid is a protective agent against liver toxicity caused by some drugs. In the present pilot study, we assessed the effect of this bile acid on tacrine-induced hepatotoxicity. Fourteen patients with a diagnosis of Alzheimer's disease received tacrine and ursodeoxycholic acid (13 mg/kg/day) for 105 days. Serum ALAT was the main evaluation criterion. Serum levels of ALAT were compared with those of 100 patients who had been treated with tacrine in the same centre. In patients receiving ursodeoxycholic acid, ALAT serum levels were normal in 93 per cent of cases vs. 69 per cent in control patients and moderate hepatotoxicity (ULN < ALAT < 3 ULN) did not occur while it was present in 25 per cent of controls (p = 0.036). In contrast, the percentage of patients with ALAT > 3 ULN was similar in the two groups (7 per cent vs. 6 per cent). These present findings suggest that UDC could prevent moderate tacrine-induced hepatotoxicity. These results should be confirmed in a controlled therapeutical trial.