Background <p>Intravenous ferric carboxymaltose (<i>FCM</i>) is frequently used to treat iron deficiency in lung transplant recipients (<i>LTRs</i>). Hypophosphatemia is a known adverse event after FCM administration, with reported significant hypophosphatemia following FCM administration in renal and heart transplant recipients. This study aimed to assess the incidence of hypophosphatemia after FCM administration and to identify potential risk factors in LTRs.</p> Methods <p>This retrospective, single-center study included LTRs treated with FCM during post-transplant follow-up between 2015 and 2022 at the University Hospital Zurich. Patients were included if serum phosphate was measured within six months before and after FCM infusion. Phosphate levels and clinical data were extracted from electronic records, and hypophosphatemia was classified by severity. Changes in phosphate levels over time were analyzed using mixed-effects models, and multivariable logistic regression was used to identify predictors of hypophosphatemia. Of note, phosphate measurements were obtained at routine follow-up visits, with the first post-infusion measurement occurring at a median of 32 days.</p> Results <p>Among 110 LTRs receiving FCM, 54.5% (<i>n</i> = 60) developed hypophosphatemia (≤ 0.81 mmol/L). Mild, moderate, and severe hypophosphatemia occurred in 23.6% (<i>n</i> = 26), 24.5% (<i>n</i> = 27), and 6.4% (<i>n</i> = 7) of patients, respectively. Use of 1000 mg FCM was associated with a higher risk of hypophosphatemia compared to 500 mg (OR (95% CI) of 2.54 (1.15–5.59)), increasing further after adjusting for baseline phosphate (OR (95% CI) of 5.18 (1.90–14.14)). Among patients with new-onset hypophosphatemia, 76% recovered within a median of 56 days. No serious clinical adverse events directly attributable to hypophosphatemia were documented.</p> Conclusions <p>Hypophosphatemia frequently occurred after FCM infusion in LTRs, particularly following higher doses. Transplant physicians should be mindful of this risk and consider using individualized FCM doses or alternative intravenous iron formulations with lower hypophosphatemia risk, such as ferric derisomaltose&#xa0;in this population.</p> Trial registration <p><a href="http://www.ClinicalTrials.gov">www.ClinicalTrials.gov</a>, NCT 06112236</p>