A common complication of chimeric antigen receptor (CAR)-T cell therapy is immune effector cell associated neurotoxicity syndrome (ICANS), which presents with encephalopathy, aphasia, inattention, somnolence, seizures, weakness, or cerebral edema. Despite its significant morbidity, there are currently no effective targeted treatments. Given the clinical similarities between ICANS and the neurological manifestations of acute hypophosphatemia, we retrospectively reviewed 499 patients treated with CD19-targeted CAR-T cell therapy across multiple clinical trials between 2015 and 2020. In addition to clinical toxicities experienced by the patients, we also interrogated the impact of serum electrolyte data and repletion of corresponding electrolyte deficiencies with ICANS incidence, severity, and duration. Hypophosphatemia was a common occurrence in CAR T-cell recipients, and the only electrolyte derangement associated with a significantly higher cumulative incidence of ICANS. Moreover, phosphorus repletion in hypophosphatemic patients was associated with significantly decreased ICANS incidence and duration. Hypophosphatemia was uniquely associated with encephalopathy neurological adverse events, which also showed the strongest positive correlation with both ICANS and CRS severity. These findings suggest that serum phosphorus could be a reliable biomarker for ICANS, and expeditious, goal-directed phosphorus repletion in response to serum hypophosphatemia could be a safe, inexpensive, and widely available intervention for such patients.