Malignant tumors of the sweat glands are exceedingly rare and aggressive tumors. We present here a case of a 60-year-old man with a malignant hidradenocarcinoma that developed in a background of preexisting benign hidradenoma on the lateral aspect of the knee that was initially resected, but rapidly recurred with associated inguinal lymphadenopathy. F-FDG PET/CT was performed as part of preoperative staging, which demonstrated abnormal inguinal lymph nodes and metastatic disease to the lungs. FDG PET/CT can play an invaluable role in the initial staging and follow-up of this rare malignancy.