The goal of the present study was to analyze the expression of <i>Ido</i> (indoleamine-2,3-dioxygenase), <i>Kat1</i> (kynurenine aminotransferase 1), <i>Kat2</i> (kynurenine aminotransferase 2), and <i>Kmo</i> (kynurenine-3-monooxygenase) genes in patients undergoing kidney failure with kidney replacement therapy (KFRT) treatment with either hemodiafiltration (HDF) or hemodialysis (HD) in relation to selected clinical and dialysis parameters. The gene discussed is AADAT; the disease is kidney failure.