BACKGROUND Pyoderma gangrenosum (PG) is a sterile neutrophilic dermatosis that can be associated with systemic diseases, such as ulcerative colitis, polyarthritis, diabetes mellitus, myelodysplastic syndrome, and/or myeloid leukemia, and is often misdiagnosed as a necrotizing infection. Few reports have described imaging studies of PG; however, necrotizing fasciitis (NF) exhibits distinct imaging characteristics. If deep fascial involvement is not demonstrated on magnetic resonance imaging (MRI), NF is excluded. CASE REPORT We present a case of PG mimicking NF on MRI in a 67-year-old woman with acute myeloblastic leukemia. After undergoing a second cycle of decitabine therapy, she was admitted for pain in her lower left leg. The condition was initially misdiagnosed as NF because MRI findings demonstrated signal intensity in the fascia. MRI revealed fasciitis that exhibited linear fluid signal intensity in the fascia of lower left leg. Despite broad-spectrum antibiotics, the lesion rapidly progressed to a swollen hemorrhagic patch with bullae and an ulcer. Skin biopsy results ultimately led to the diagnosis of PG, based on histopathological findings. The patient was treated with intravenous steroids and regular wound dressing. The skin lesion on the lower left leg exhibited a good response. CONCLUSIONS Despite the presence of a lesion that invaded the fascia on MRI, our patient was diagnosed with PG following a skin biopsy and completely recovered with steroid treatment. To distinguish PG from NF, it is more important to identify the characteristic clinical features than to rely solely on imaging findings.