A 74-year-old man was admitted for chest infection with acute exacerbation of chronic obstructive pulmonary disease. He was incidentally found to have an increased serum level of cardiac troponin I, despite the absence of symptoms and electrocardiographic evidence of ischaemic heart disease. Troponin I became undetectable after the serum was treated with polyethylene glycol, which removed any interfering antibodies. Serum cardiac troponin T was also undetectable after this treatment. Interference of the cardiac troponin I assay by heterophilic antibodies was thus confirmed. Because of the possibility of false-positive results due to immunoassay interference, clinicians should be alerted whenever laboratory findings are incompatible with the clinical picture, and should be ready to perform additional laboratory tests.