The diagnostic efficacy of five serum liver function tests (aspartate and alanine aminotransferase, alkaline phosphatase, 5' nucleotidase, and bilirubin) was investigated in 95 bone marrow transplant recipients in whom acute graft-vs-host disease was graded by the Seattle criteria. The patient population included a control group of 22 autologous transplant recipients (group I), 33 patients with no GVHD (group II), 21 patients with grades 1 and 2 GVHD (group III), 12 patients with grade 3 GVHD (group IV), and 7 patients with grade 4 GVHD (group V). Studentttest analysis of the analytes among the five groups of patients showed that 5' nucleotidase and alkaline phosphatase were the best discriminants among all the possible combinations of group pairs. Peak levels of 5' nucleotidase within each group of patients correlated well with those of alkaline phosphatase in all the allogeneic transplant groups (II-V; r = 0.59), but the correlation of these with bilirubin was less frequent. Also, 5' nucleotidase and alkaline phosphatase showed significant discrimination (P< 0.05) even between groups I and II, suggesting that they are more sensitive than the Seattle criteria in the diagnosis of GVHD. They also showed the best overall discriminatory ability by one-factor analysis of variance (ANOVA;P= 0.0001 as compared with 0.002, 0.009, and 0.04 for aspartate aminotransferase, alanine aminotransferase, and bilirubin, respectively). Receiver-operating curves of the five analytes again revealed that 5' nucleotidase and alkaline phosphatase were by far the best discriminators among the five groups of patients. Bilirubin was relatively insensitive because it was a good discriminator only between the control group and groups IV and V. The hepatocellular enzymes, alanine and aspartate aminotransferase, correlated well (r = 0.80) but discriminated poorly among the four groups of allogeneic transplant recipients (II-V), suggesting that all four groups had some measure of hepatocellular damage that was independent of the severity of GVHD.