Peritoneal cestodiasis is a parasitic disease, well described in the veterinary literature, consisting of exudative peritonitis caused by larval stage (Tetrathyridium) proliferations of Mesocestoides spp. This disease syndrome has been termed canine peritoneal larval cestodiasis (CPLC). The disease is unusual in Italy and the only cases reported were in Northern Italy. This report describes a case of asymptomatic peritoneal cestodiasis in Central Italy (Rome). A 7-year-old, male, cross-breed dog was referred for abdominal ultrasound for a severe asymmetric testicle hyperplasia. The images revealed peritoneal reactivity with diffuse abdominal cystic structures and diffuse hyperechoic fluid. Abdominocentesis detected the presence of lactescent fluid. The cytological examination was consistent with neutrophilic peritonitis associated with peritoneal cestodiasis.