Erythromycin acistrate is a new 2'-acetyl esther prodrug of erythromycin, whose structure resembles that of erythromycin estolate. However, in toxicological studies, it does not have the problems of hepatotoxicity. To assess its effects on hepatic functions in clinical practice, the liver parameters of patients with respiratory tract or skin infections were monitored during therapy. In total 1549 patients were treated for 7-14 days. In addition, 127 patients with suspected viral infections served as controls. There were no significant differences in serum aspartate aminotransferase (ASAT), alanine aminotransferase (ALAT), gamma-glutamyltransferase (gamma-GT) or alkaline phosphatase (APHOS) values between the erythromycin acistrate or control groups at the beginning or end of therapy. ASAT values increased moderately in 2.4% and clearly in 0.3% of patients treated, but also decreased in 2.0%. ALAT values were moderately increased in 9.9%, clearly increased in 0.6% and normalized in 3.5% of the patients. gamma-GT values increased moderately in 3.5% and and clearly in 0.3%, but decreased to normal in 3.3% of the patients. APHOS was moderately elevated in 1.0% of the patients and normalized in 1.3%. The correlation of changes between the different liver enzymes was poor. Only ten patients (0.6%) had two or more clearly elevated liver enzyme values by the end of the therapy, of whom five had increased liver enzyme activities before the treatment, two had underlying disease explaining the changes and in only three patients out of 1549 (0.2%) could hepatic changes be attributed to erythromycin acistrate therapy. These changes were reversible. The results demonstrate the hepatic safety of erythromycin acistrate in clinical practice. Concomitant food intake did not affect the safety profile.