Objective: The aim of this study was to evaluate the usefulness of ultrasonography in the diagnosis of hemorrhagic cystitis following bone marrow transplantation in children. Material and methods: The study involved an analysis of clinical material and the results of imaging tests performed in 334 patients who underwent hematopoietic cell transplantation. Ultrasonographic findings in 42 patients with hemorrhagic cystitis were analyzed in detail. The ultrasound images served to assess the severity of hemorrhagic cystitis and the results were compared with the clinical assessment of the disease on the Droller scale, as well as the laboratory and endoscopic tests. Results: In the studied group of patients hemorrhagic cystitis following allogeneic transplantation was diagnosed in 12.5% cases. 73.8% patients received transplants from unrelated donors, 26.2% – from compatible siblings. The study revealed a higher incidence of hemorrhagic cystitis in children above 10 years of age. Grade 3 according to the Droller was diagnosed in 42.9%, grade 2 – in 30.9%, grade 4 – in 14.3%, and grade 1 – in 11.9% patients. The number of ultrasound examinations depended on the clinical symptoms, severity, duration and co-occurrence of other complications following the transplantation and was within the 1–15 range (average: 4.6). Grades 3 and 4 were related to the poor clinical condition of the patients and to their longer hospitalization. During this period there was an increased risk of renal malfunction and acute renal failure, post-inflammatory narrowing of the ureters, hydronephrosis, and in grade 4 the fibrosis of the bladder with reduced bladder capacity. Analyses demonstrated a significant correlation between the ultrasound image of the bladder wall and the clinical severity. Conclusions: Ultrasound with Doppler options remains the primary diagnostic tool in the evaluation of hemorrhagic cystitis, and is useful in terms of its diagnosis, determination of the severity, and monitoring of the treatment.