Aspartate aminotransferase (AST) macroenzyme leads to an increase in AST without the presence of any disease. In present report, an isolated increase of AST was detected in a 6-year old girl following the investigations prior to tonsillectomy. As to her history, physical examination and other laboratory test results, no abnormal findings were detected. AST levels were measured as 591 IU/L and 585 IU/L after repeated tests. While found as <5.4 IU/L (recovery <0.8%, reference interval 42.0-82.2%) after polyethylene glycol (PEG) precipitation, AST was determined as 561 IU/L (recovery 95.4%) as a result of the assessment performed with non-specific human antimouse antibodies (HAMA). No diseases were encountered in the patient, and reason for the increase of AST was considered to be macroAST. Because continuously increased enzyme value may lead to various invasive and expensive diagnostic tests, macroAST should be taken into account in the evaluation process.