The purpose of the study was to determine the effect of adjuvant chemotherapy on liver enzymes in breast cancer patients. Furthermore, the effect of tamoxifen on liver enzymes was analyzed. Liver function tests from 194 breast cancer patients who received adjuvant chemotherapy with or without tamoxifen (TAM) were reviewed. Statistically very significant increases were seen in alkaline phosphatase, aspartate acetyl transferase, and gamma glutamyl transferase levels in these patients receiving adjuvant chemotherapy. No statistical changes were noticed in bilirubin levels. If tamoxifen was given together with adjuvant chemotherapy, no changes in liver function tests were detected. Hepatic toxicity was induced in breast cancer patients by adjuvant CMF/CNF therapy (cyclophosphamide, methotrexate, 5-fluorouracil, mitoxantrone). These changes were mostly mild. Adjuvant tamoxifen reduced the increase in liver enzymes caused by adjuvant chemotherapy.