We report on the search for low molecular weight molecules-possibly accumulated in the bloodstream and body-in the exhaled breath of uremic patients with kidney malfunction. We performed non-invasive analysis of the breath gas of 96 patients shortly before and several times after kidney transplantation using proton-transfer-reaction mass spectrometry (PTR-MS), a very sensitive technique for detecting trace amounts of volatile organic compounds. A total of 642 individual breath analyses which included at least 41 different chemical components were carried out. Correlation analysis revealed one particular breath component with a molecular mass of 114 u (unified atomic mass units) that clearly correlated with blood serum creatinine, which is the currently accepted marker for assessing the function of the kidney. In particular, daily urine production showed good correlation with the identified breath marker. An independent set of seven samples taken from three patients at the onset of dialysis and three controls with normal kidney function confirmed a significant difference in concentration between patients and controls for a compound with a molecular mass of 114.1035 u using high mass resolving proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS). A chemical composition of C7H14O was derived for the respective component. Fragmentation experiments on the same samples using proton-transfer-reaction triple-quadrupole tandem mass spectrometry (PTR-QqQ-MS) suggested that this breath marker is a C7-ketone or a branched C7-aldehyde. Non-invasive real-time monitoring of the kidney function via this breath marker could be a possible future procedure in the clinical setting.