Abstract Beside their pharmacological action drugs may have unexpected side effects which must be known and taken into account in the interpretation of laboratory tests. In this work, we followed 17 hyperlipidemic subjects, aged 4–19 years, treated with fenofibrate only. We performed several plasma tests before and during treatment. The hypolipidemic action of fenofibrate was evident: decreases of cholesterol and triglycerides mean values were, respectively, 22% and 39% after 3 months of treatment. Other plasma constituents were also significantly decreased: uric acid, 20%; bilirubin, 19%; alkaline phosphatases, 15%. The aminotransferase activities (both AST and ALT) were increased in only four subjects after 3 months. The highest values were 88 U/l for AST and 109 U/l for ALT (the initial values were, respectively, 28 U/l and 23 U/l). These activities decreased when the treatment was stopped. These variations were isolated and transient.