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Background: Urinary tract infection is the most common infectious disease requiring hospitalisation following renal transplantation. However, the risk and outcome of post-transplant pyelonephritis remains unclear.Methods: This population-based cohort study was conducted from 1 January 1990 to 31 December 2009. Each member of a Danish population-based, nationwide cohort of first-time renal transplant recipients was matched by age and gender with up to 19 population controls. Information on hospital discharge diagnosis, emigration, and mortality was obtained from nationwide administrative databases. Individuals were observed from the date of first renal transplantation and until graft loss, emigration, or death. Risk factors were assessed by Poisson regression.Results: The incidence rate (IR) of first-time hospitalisation for pyelonephritis was 18.5 (95 % confidence interval [CI]: 16.4-20.9) per 1,000 person-years of follow-up (PYFU) among renal transplant recipients (N = 2,656) and 0.26 (CI: 0.21-0.31) per 1,000 PYFU among population controls (N = 49,226) yielding an incidence rate-ratio (IRR) of 72.0 (95 % CI: 57.8-89.7). Among renal transplant recipients, the risk of pyelonephritis decreased during the entire study period and was lowest in 2005-09 (IRR = 0.46, CI: 0.31-0.68). The highest risk of pyelonephritis was observed within the first six months post-transplantation (IR = 69.9 per 1,000 PYFU; CI: 56.4-86.7). Other risk factors for post-transplant pyelonephritis included female gender, high Charlson comorbidity index score, HLA-DR mismatch, cause of renal failure, and calendar period. Interestingly, we found that the combined risk of graft loss and death was 45 %, (CI: 19-77 %) higher in renal transplant recipients following post-transplant pyelonephritis compared to those who had no admission due to pyelonephritis.Conclusions: The risk of first-time hospitalisation for pyelonephritis among renal transplant recipients is high. Further, post-transplant pyelonephritis was associated with excess risk of graft loss and death; this indicates that strategies aimed at reducing upper urinary tract infections are likely to enhance renal graft survival. [ABSTRACT FROM AUTHOR]