Pathological conditions involving the lesser sac of the peritoneal cavity in patients on peritoneal dialysis (PD) can pose significant diagnostic and therapeutic challenges. Lack of appreciation of these challenges may delay diagnosis and compromise outcome. A case series by Li and colleagues in this issue of Peritoneal Dialysis International highlights the diagnostic challenges presented by lesser sac infection in PD patients, and in this accompanying commentary we discuss the development and anatomy of the lesser sac, as well as the pathological conditions and investigations relevant to the management of patients on PD.