BACKGROUND: Optimal daily water intake to prevent chronic kidney disease (CKD) progression is unknown. Taking kidney urine-concentrating ability into account, we studied the relation of kidney outcomes in patients with CKD to total and plain water intake and urine volume. METHODS: Including 1265 CKD patients (median age, 69 years; mean estimated glomerular filtration rate [eGFR], 32 ml/min per 1.73 m2) from the CKD-REIN cohort (2013-2019), we assessed fluid intake at baseline interviews, collected 24-h urine volumes, and estimated urine osmolarity (eUosm). Using Cox and then linear mixed models, we estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for kidney failure and eGFR decline associated with hydration markers, adjusting for CKD progression risk factors and eUosm. RESULTS: Patients' median daily intake was 2.0 (interquartile range: 1.6-2.6) L total water and 1.5 (1-1.7) L plain water; median urine volume was 1.9 (1.6-2.4) L/24 h, and mean eUosm, 374 ± 104 mosm/L. Neither total water intake nor urine volume was associated with either kidney outcome. Kidney failure risk increased significantly with decreasing eUosm below 292 mosm/L. Adjusted HRs for kidney failure associated with plain water intake were 1.88 (95%CI 1.02; 3.47), 1.59 (1.06; 2.38), 1.76 (0.95; 3.24), and 1.55 (1.03; 2.32) in patients drinking \textless0.5, 0.6-1.0, 1.6-2.0, and \textgreater2.0 L/day, compared with those drinking 1.0-1.5 L/day. High plain water intake was also significantly associated with faster eGFR decline. CONCLUSION: In patients with CKD, the relation between plain water intake and progression to kidney failure appears to be U-shaped. Both low and high intake may not be beneficial in CKD.