Changes in liver biochemical test results have been described in hyperthyroid patients before and after antithyroid therapy. In the present study, we analyzed liver tests at diagnosis and after 6 weeks of treatment with propylthiouracil (PTU) in 43 patients with hyperthyroidism. At diagnosis, 60.5% of the patients had at least one liver abnormality. Elevations of alkaline phosphatase, alanine and aspartate aminotransferase, and gamma-glutamyl transpeptidase levels were observed in 19 (44.2%), 10 (23.3%), six (14%), and six (14%) of the patients, respectively. At the end of the 6-week treatment with PTU, elevations in liver test values, possibly induced by PTU, were found in seven (16.3%) patients. Age, sex, type of goiter (either diffuse or multinodular), and presence or absence of abnormal liver biochemical tests at diagnosis were not significant in determining the possibility of PTU-induced elevations in liver tests. These data suggest that liver test abnormalities are frequently found at the time of diagnosis of hyperthyroidism. However, the presence or absence of these abnormalities does not predict elevations in liver test results, which are possibly induced by PTU during therapy.