A 72-year-old patient being treated with acalabrutinib for blood cancer developed a slowly growing skin ulcer on the thigh. Testing identified a fungal infection caused by Alternaria alternata, a mold found in soil and air. The patient was successfully treated with an antifungal medication called posaconazole, with the ulcer healing over one month. This case highlights that doctors should check for fungal infections in chronic skin wounds of patients taking BTK inhibitor medications.