BACKGROUND : The relationship between acute kidney injury (AKI) and long-term renal function is controversial. The influence of AKI duration on functional recovery after partial nephrectomy has never been investigated. OBJECTIVE : To investigate the association between AKI and renal function 1yr after partial nephrectomy, and whether this relationship is affected by the duration of AKI. METHODS : We analyzed the data of 1893 patients treated by partial nephrectomy for a single cT1 N0 M0 renal mass. UNASSIGNED : We defined three outcomes of interest: (1) recovery of at least 90% of baseline function 1yr after partial nephrectomy, (2) percentage change of 1-yr renal function compared with baseline function, and (3) chronic kidney disease (CKD) upstaging. AKI was defined according to the RIFLE criteria and recorded up to the 7th postoperative day. The association between AKI and each endpoint of interest was examined using regression models after adjustment for common predictors of renal function. CONCLUSIONS : A total of 388 (20%) patients experienced AKI after surgery. The rate of patients recovering 90% of baseline function was lower in the AKI group (30% vs 61%), while the proportion of patients who had CKD upstaging was significantly higher (51% vs 23%; both p<0.0001). At multivariable analysis, AKI was associated with worse renal function 1yr after partial nephrectomy, regardless of the outcome of interest (all p<0.0001). Longer AKI increases the risk of functional deterioration, especially after the 3rd day of injury. The risk of CKD upstaging for an average patient who had 1-3 versus ≥4 d of AKI was 46% (95% confidence interval [CI]: 40%, 52%) versus 67% (95% CI: 55%, 78%; absolute risk increase of 21%; 95% CI: 8%, 34%). CONCLUSIONS : AKI negatively affects long-term functional recovery after partial nephrectomy, and thus, modifiable factors associated with AKI should be identified and corrected preoperatively. The duration of injury is informative, and should be included in the assessment of AKI and in future studies addressing this topic. UNASSIGNED : Proper functional recovery after partial nephrectomy is jeopardized by acute kidney injury (AKI). Inclusion of the dimension of time into classification systems for AKI may be beneficial for postoperative risk stratification.