Persistent fever in a 60-yr-old man with polycystic kidney and liver diseases and bilateral hip prostheses was presented in this study. Multiple diagnostic tests failed to localize a source of infection. Subsequently, a combination of a 111In-oxine labeled WBC and 99mTc-sulfur colloid scans (and computer subtraction) demonstrated abnormally increased WBC activity in the left lobe of the liver, thus, diagnosis of an infected cyst (or cysts) was made. The patient responded to the treatment with antibiotics. This article discusses the clinical features of polycystic disease of the liver and kidneys. Infection in cysts are discussed as well as radiographic and scintigraphic investigations that can be used to diagnose and localize infection in a cyst.