AbstractAltered tryptophan–kynurenine (KYN) metabolism has been implicated in major depressive disorder (MDD). The l-[1-¹³C]tryptophan breath test (¹³C-TBT) is a noninvasive, stable-isotope tracer method in which exhaled ¹³CO₂ is attributable to tryptophan catabolism via the KYN pathway. We included 18 patients with MDD (DSM-IV) and 24 age- and sex-matched controls. ¹³C-tryptophan (150 mg) was orally administered and the ¹³CO₂/¹²CO₂ ratio in the breath was monitored for 180 min. The cumulative recovery rate during the 180-min test (CRR_0–180; %), area under the Δ¹³CO₂-time curve (AUC; %*min) and the maximal Δ¹³CO₂ (C_max; %) were significantly higher in patients with MDD than in the controls (p = 0.004, p = 0.008 and p = 0.002, respectively). Plasma tryptophan concentrations correlated negatively with C_max in both the patients and controls (p = 0.020 and p = 0.034, respectively). Our results suggest that the ¹³C-TBT could be a novel biomarker for detecting a subgroup of MDD with increased tryptophan–KYN metabolism.