We determined the optimum 13C-labeling position in phenacetin for use in a breath test to diagnose liver disease based on infrared spectroscopy detection of 13CO2 in exhaled air. ([1-13C]Ethoxy)phenacetin gave the best result. This compound was also employed in a urine test using 13C-NMR spectroscopy. In the urine test, healthy subjects gave a higher signal of phenacetin than of its metabolite, phenetidine, whereas in patients with liver disease the situation was the reverse. The combination of the breath and urine tests may be a valuable new tool for the diagnosis of liver disease.