OBJECTIVE: The relationship between serum creatinine and calcium (Ca) was investigated in hematopoietic stem cell transplantation (HSCT) patients treated with foscarnet. MATERIALS AND METHODS: A retrospective study was performed to investigate the development of foscarnet-induced renal dysfunction in patients who received HSCT from April 2010 to November 2018 at the Kindai University Nara Hospital. A total of 80 patients were identified from the medical records, and 42 patients who met the inclusion criteria were enrolled in this study. Renal dysfunction was classified according to the Kidney Disease: Improving Global Outcomes (KDIGO) criteria. RESULTS: A significant inverse relationship was observed between serum creatinine and Ca levels (r = -0.372; p < 0.0001; y = -0.537x + 9.268). A separate analysis divided into renal dysfunction and non-renal dysfunction groups showed that there was a significant relationship between serum creatinine and Ca levels in the renal dysfunction group (r = -0.531; p < 0.0001; y = -0.617x + 9.239) but not in the non-renal dysfunction group (r = -0.011; p = 0.561; y = -0.023x + 8.934). The optimal cutoff for the minimum Ca level was calculated to be 8.1 mg/mL. CONCLUSION: A significant inverse relationship was observed between serum creatinine and Ca levels in HSCT patients with foscarnet-induced renal dysfunction. Foscarnet-induced renal dysfunction should be noted if Ca levels fall below 8.1 mg/dL. Monitoring Ca levels may be useful for detecting renal dysfunction at early stages in patients treated with foscarnet.