We report here a case of pyoderma gangrenosum in a patient with myelodysplastic syndrome developing into myeloid sarcoma as a sign of transformation to acute leukaemia. The patient was treated successfully with intensive chemotherapy and achieved complete remission, and her otherwise expanding ulcers started to heal. This is the first reported case of secondary blastic infiltration in pyoderma gangrenosum, and it underlines the importance of performing re-biopsy of non-healing ulcers, especially in patients with an underlying haematological disease.