Hepatic and/or renal cyst infection is a major complication in patients with polycystic kidney disease. In many cases, drainage of infected cysts is necessary, although accurate detection of infected cysts from among the numerous hepatic or renal cysts present is often difficult, because the findings of infected cysts on computed tomography and T1- and T2-weighted magnetic resonance imaging resemble those of normal cysts. We describe here a case of polycystic kidney disease complicated by hepatic cyst infection. On diffusion-weighted magnetic resonance imaging (DWMRI), which is occasionally used in the diagnosis of cerebral abscesses, infected hepatic cysts showed higher signal intensity than other cysts, facilitating differentiation of the cysts requiring drainage from numerous other cysts. Infected cysts showed a marked decrease of the apparent diffusion coefficient (ADC) values compared with those of normal cysts. DWMRI was very effective in detecting infected cysts in our patient and may be of value in other such cases with polycystic kidney disease.