Background: Drug interactions (DI) in patients receiving hematopoietic stem cell transplantation (HSCT) are common and clinically significant, highlighting: anticonvulsants, voriconazole (VCZ) and cyclosporine (CsA), which require monitoring. Objective: To describe the interactions between CsA-VCZ in children undergoing HSCT. Methods: Retrospective, descriptive study in immunocompromised children hospitalized since January 2013 to December 2014 at Bone Marrow Transplant Unit, Hospital Dr. Luis Calvo Mackenna, who received CsA and VCZ. Results: The median age was 5 years (3-6) and the median weight was 20 kg (17-30). Sixtythree baseline drug levels were analyzed, of those, 27 were CsA drug levels obtained previous to using VCZ and 36 were CsA drug levels collected concomitantly with VCZ. In the group CsA previous to VCZ, the CsA dose was 4.6 +/- 2.6 (mg/kg/day) and the CsA average level was 188.8 +/- 84.1 (mu g/ml). In the group of CsA concomitantly with VCZ, the dose of CsA was 5.5 +/- 3.0 (mg/kg/day) (p = 0.07) and CsA average level was significantly higher: 232.5 +/- 106.7 (mu g/ml) (p = 0.04). Conclusion: This study shows an increased level of CsA when it is used together with VCZ. Therapeutic drug monitoring could improve the management of the DI and optimize the co-administration of CsA and VCZ.