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Abstract Background Isavuconazole (ISA) is a newer antifungal used in patients with history of hematologic malignancies and hematopoietic transplant and cellular therapies (HM/TCT). Although it has a more favorable side‐effect profile, breakthrough invasive fungal infections (bIFIs) while on ISA have been reported. Methods In this single‐center retrospective study evaluating HM/TCT patients who received prophylactic ISA for ≥7 days, we evaluated the incidence and potential risk factors for bIFIs. Results We evaluated 106 patients who received prophylactic ISA. The patients were predominantly male (60.4%) with median age of 65 (range: 21–91) years. Acute myeloid leukemia (48/106, 45.3%) was the most common HM, with majority having relapsed and/or refractory disease (43/106, 40.6%) or receiving ongoing therapy (38/106, 35.8%). Nineteen patients (17.9%) developed bIFIs–nine proven [ Fusarium (3), Candida (2), Mucorales plus Aspergillus (2), Mucorales (1), Colletotrichum (1)], four probable invasive pulmonary Aspergillus , and six possible infections. Twelve patients were neutropenic for a median of 28 (8–253) days prior to bIFI diagnosis. ISA levels checked within 7 days of bIFI diagnosis (median: 3.65 μg/mL) were comparable to industry‐sponsored clinical trials. All‐cause mortality among the bIFI cases was 47.4% (9/19).We also noted clinically significant cytomegalovirus co‐infection in 5.3% (1/19). On univariate analysis, there were no significant differences in baseline comorbidities and potential risk factors between the two groups. Conclusion ISA prophylaxis was associated with a significant cumulative incidence of bIFIs. Despite the appealing side‐effect and drug‐interaction profile of ISA, clinicians must be vigilant about the potential risk for bIFIs. image