BACKGROUND: Tryptophan is metabolized along the kynurenine pathway, initially to kynurenine, and subsequently to cytotoxic 3-hydroxykynurenine. There is increasing interest in this pathway, because of its pro-inflammatory nature, and drugs interfering in it receive increasing attention. We aimed to investigate whether serum and urinary parameters of the tryptophan-kynurenine pathway, and particularly cytotoxic 3-hydroxykynurenine, are associated with systemic inflammation and long-term outcome in renal transplant recipients (RTR). METHODS: Data were collected in outpatient RTR with a functioning graft for >1 year. Tryptophan, kynurenine and 3-hydroxykynurenine in serum and urine were measured using LC-MS/MS. RESULTS: A total of 561 RTR (age 51±12 years; 56% male) were included at median 6.0 [2.6-11.6] years post-transplantation. Baseline median serum tryptophan was 40.0 [34.5-46.0] µmol/l; serum kynurenine was 1.8 [1.4-2.2] µmol/l; serum 3-hydroxykynurenine was 42.2 [31.0-61.7] nmol/l. Serum kynurenine and 3-hydroxykynurenine were strongly associated with parameters of systemic inflammation. During follow-up for 7.0 [6.2-7.5] years, 51 RTR (9%) developed graft failure and 120 RTR (21%) died. Both serum kynurenine and 3-hydroxykynurenine were independently associated with graft failure (HR 1.72 [1.23-2.41], P=0.002 and HR 2.03 [1.42-2.90], P<0.001). Serum 3-hydroxykynurenine was also independently associated with mortality (HR 1.37 [1.08-1.73], P=0.01), while serum kynurenine was not. Urinary tryptophan-kynurenine pathway parameters were not associated with outcome. CONCLUSIONS: Of tryptophan metabolites, serum 3-hydroxykynurenine is cross-sectionally most strongly and consistently associated with systemic inflammation and prospectively with adverse long-term outcome after kidney transplantation. Serum 3-hydroxykynurenine may be an interesting biomarker and target for the evaluation of drugs interfering in the tryptophan-kynurenine pathway.