The metabolic basis and clinical application of the aminopyrine breath test (ABT) as a measure of liver function is reviewed in this article. Several papers have been published in the 20 years that have elapsed since the test was validated in man by Hepner and Vesell. Nevertheless, even if the aminopyrine breath test has been shown to be a non-invasive, reliable and semiquantitative liver function test with diagnostic and prognostic accuracy, it is not yet extensively used in clinical practice, probably because it is not widely known to clinicians. The aminopyrine breath test, like other newer tests (phenacetin, caffeine and erythromycin breath tests), allows the effects of drugs on hepatic cytochrome P-450 to be explored both in normal subjects and in liver patients. This interesting field of application is sure to expand the appeal of the aminopyrine breath test in the future.