The lidocaine-monoethylglycinexylidide (MEGX) test is used to monitor liver function in liver transplant recipients. Serial studies have been undertaken after 155 allografts. The initial MEGX concentration is significantly correlated with the donor MEGX concentration. It is also influenced by the recipient's pretransplant bilirubin concentration, being lowest among patients with very high bilirubin levels. Use of segmental grafts is also accompanied by low MEGX concentrations. The flow-dependent clearance of lidocaine makes it a sensitive indicator of disturbed liver blood flow, with decreased MEGX concentrations occurring in hepatic artery thrombosis and rejection and as a result of cardiac failure and pulmonary effusions. Significant hepatic ischemia resulting in delayed initial function or cholestasis also is associated with low MEGX concentrations. The initial median MEGX concentrations were lowest among patients who required retransplantation or who died within 2 months of allografting.