Abstract Background and Aims Chimeric antigen receptor T cell therapy (CAR-T) has improved the prognosis of patients with refractory hematologic malignancies. The most important toxicities are the cytokine release syndrome (CRS), where interleukin 6 (IL-6) plays a major role, and the immune effector cell-associated neurotoxicity syndrome (ICANS). A low incidence of acute kidney injury (AKI) with a high frequency of electrolyte imbalance has been described with CAR-T therapy. The aim of our study is to describe the nephrotoxicity of CAR-T therapy and to focus on AKI and the physiopathology of hypophosphatemia. Method A prospective single-center case series was performed which included all patients undergoing CAR-T therapy between 2020 and 2022. Clinical data included medical history, previous chemotherapy treatments, concomitant treatments, and clinical evolution. Daily monitoring of renal function, electrolytes and IL-6 was obtained after infusion of CAR-T. A urine sample was obtained on days 0, +7 and +14 with parathyroid hormone (PTH) and 25-OH vitamin D. AKI was classified according to the KDIGO criteria. Results 40 patients received CAR-T therapy during the follow-up period. The median age was 62 years. The same number of men as women were included. The most frequent comorbidities were hypertension (27.5%), followed by diabetes (12.5%) and chronic kidney disease (12.5%). 12 patients presented AKI. Baseline serum creatinine (SCr) in patients with AKI was 0.9 mg/dL (0.68-1.04) with a SCr peak of 1.27 mg/dL (1.1-1.7). The time from CAR-T therapy infusion to SCr peak was 6.5 days (4-10). Of these 12 patients, 8 presented stage 1, 3 stage 2, and 1 stage 3 AKI. There were no differences between the AKI and non-AKI group in baseline characteristics, inflammatory markers, CRS, ICANS or hospital length stay. Hypophosphatemia was the most frequent electrolyte imbalance, occurring in 36 patients (90 %), 22 (61 %) patients experienced moderate hypophosphatemia (phosphorus < 2 mg/dL) while 14 (39 %) patients experienced mild hypophosphatemia. 29 patients (72 %) presented hypokalemia and 25 patients (62.5 %) hyponatremia. Patients with moderate hypophosphatemia had a significantly higher elevation of IL-6 but there were no differences in PTH or vitamin D (Table 1). Although there were no significant differences in fractional excretion of filtered phosphate (FEPO4), patients with moderate hypophosphatemia trend to have higher FEPO4 values. Conclusion Electrolyte disturbances are common in CAR-T therapy, with hypophosphatemia being the most frequent. There is probably a mechanism for renal phosphorus loss related to systemic inflammation. AKI is less frequent and has mild characteristics in these patients.