A 43-year old man presents with acute febrile neutrophilic dermatosis (Sweet's syndrome) associated with acute seronegative polyarthritis. Although no definite diagnosis of viral infection could be made, the patient had raised serum antibodies against cytomegalovirus (1/1280; 1/2560). Histological examination showed typical lesions of Sweet's syndrome as well as the presence of extra- and intracellular unidentified particles in histiocytes.