Background <p>Hyperkalemia following nephrectomies in children is a poorly studied, potentially life-threatening complication. We describe the frequency and risk factors for hyperkalemia in pediatric patients who underwent nephrectomies resulting in an anephric state.</p> Methods <p>This was a single center retrospective cohort study of patients &lt; 21y who underwent nephrectomies resulting in an anephric state. Possible predictors included age, sex, preoperative potassium value, preoperative dialysis modality, preoperative urine output, procedure type, surgical approach, intraoperative lactated Ringer’s (LR), and intraoperative blood. The primary outcome was intraoperative and postoperative day 0 and day 1 hyperkalemia. Hyperkalemia was defined as potassium &gt; 5.1&#xa0;mEq/L (severe &gt; 5.9) in children ≥ 1y and &gt; 5.5 (severe &gt; 6.1) in children &lt; 1y. Treatments for hyperkalemia were recorded. We examined associations with hyperkalemia using Wilcoxon rank-sum test, Pearson’s Chi-squared test, or Fisher’s exact test. We used Youden’s Index to determine the preoperative potassium cutoff for predicting hyperkalemia.</p> Results <p>Among 46 patients who underwent nephrectomies resulting in an anephric state, 23 (50%) experienced hyperkalemia that was severe in 18 (39%), treated in 21 (46%), and precipitated transfer to the intensive care unit in 5 (11%). In unadjusted analysis, higher preoperative potassium (<i>p</i> &lt; 0.001) and intraoperative LR (<i>p</i> = 0.018) were associated with a higher risk of postoperative hyperkalemia. A preoperative potassium of &gt;3.85&#xa0;mEq/L was associated with the development of hyperkalemia (<i>p</i> &lt; 0.001).</p> Conclusions <p>Clinically important hyperkalemia is a common complication following nephrectomy that results in an anephric state. Targeting a preoperative potassium &lt; 3.85&#xa0;mEq/L may decrease intraoperative and postoperative hyperkalemia.</p> Graphical abstract <p></p>