No epidemiological data exist so far on invasive mucormycosis (IM) in Polish hematopoietic cell transplantation (HCT) and pediatric hemato-oncology (PHO) centers. The objective of this study was to analyze the incidence, clinical course, therapy, and outcome of IM in pediatric and adult patients undergoing HCT and children with hemato-oncological diseases in Poland. A total number of 12425 at-risk patients were retrospectively analyzed, and the period between 2010 and 2019 was included. Patients were analyzed in three groups: nontransplant children with malignancies, children undergoing HCT, and adults after HCT. Twenty-one patients were diagnosed with IM, including 15 children (10 non-HCT, 5 HCT) and 6 HCT adults. Proven IM was confirmed in 18 patients, probable in 2 patients, and possible in 1 patient. Proven IM was diagnosed in 7.1% of all patients with invasive fungal diseases (IFDs), including 8.1% among PHO patients, 5.4% among pediatric HCT patients, and 7.0% among adult HCT patients. Clinically, pneumonia was diagnosed in 10 (47.6%) patients, involvement of the paranasal sinuses was found in 3 (14.3%) patients, gastrointestinal disease was noted in 2 (9.5%) patients, and disseminated mucormycosis was found in 6 (28.6%) patients. The probability of overall survival in IM patients was 0.50 ± 0.11. Infection-related mortality (IRM) was 10% for pediatric nontransplant IM patients and 82% for transplant IM (pediatric + adult) patients ( = 0.004). Among the transplant patients, all adults died within 120 days. IRM for pediatric HCT patients was 60% ( = 0.038). The only prognostic factor was HCT, which adversely influenced survival in IM patients.