The objective of this study was to assess changes of aspartate aminotransferase (AST [serum glutamic oxalacetic transaminase]) and alanine aminotransferase (ALT [serum glutamic pyruvic transaminase]) values after a hemodialysis (HD) session. Aspartate aminotransferase and ALT serum values were measured before and after a 4 h HD session in 37 stable patients (group A), before and after a 3.5 h isovolemic HD (IVHD, with a zero ultrafiltration rate) session in eight patients (group B), as well as before and after a session of "isolated ultrafiltration" (IUF) in eight more patients (group C). In group A, mean predialysis AST and ALT serum values that were 16.4 ± 4.3 and 16.0 ± 5.5 IU/L, increased to 20.1 ± 5.4 and 17.7 ± 6.2 IU/L, respectively (p < 0.0001, comparing pre- to postdialysis values, for both). When the postdialysis values were corrected for hemoconcentration induced by ultrafiltration, no differences were found in ALT values. However, postdialysis-corrected AST values (mean 17.7 ± 4.4 IU/l) remained higher compared with predialysis values (p < 0.001). The AST/ALT ratio was 1.13 ± 0.4 for predialysis values and 1.26 ± 0.5 for postdialysis values (p < 0.0001). Predialysis mean serum AST and ALT values did not show any difference after the 3.5 h IVHD sessions (group B), but increased significantly (p < 0.02) after the removal of 1.7 ± 0.2 L of fluid by IUF (group C). In chronic HD patients, postdialysis serum AST and ALT values increased significantly compared with predialysis values. This increment is mainly due to hemoconcentration induced by ultrafiltration and cannot be attributed to the removal of an aminotransferase inhibitor by HD.