Originally described in 1964 as an idiopathic dermatosis occurring in women, Sweet's syndrome (acute febrile neutrophilic dermatosis) is associated with malignancy, typically haematological. We report a case occurring in a man receiving radical radiotherapy for locally advanced prostate cancer. To our knowledge, prostate cancer associated with Sweet's syndrome has been reported only once previously in the absence of other concurrent solid or haematological malignancy.