The risk of an abnormal liver test in 813 patients with multiple sclerosis or clinically isolated syndrome enrolled in placebo arms of clinical trials was greater than expected for alanine aminotransferase (ALT) (relative risk [RR] 3.7; 95% CI: 2.3 to 6.0) and aspartate aminotransferase (AST) (RR 2.2; 95% CI: 1.3 to 3.6), although not alkaline phosphatase (AP) or total bilirubin, at first presentation. Abnormal test results were associated with higher body mass index (ALT only), male gender (ALT only), and a relapsing-remitting (vs secondary-progressive) course (ALT and AST only).