A hydrocele (Br English: hydrocoele) denotes a pathological accumulation of serous fluid in a body cavity.
A hydrocele testis is the accumulation of fluids around a testicle, and is fairly common. It is often caused by fluid secreted from a remnant piece of peritoneum wrapped around the testicle, called the tunica vaginalis. Provided there is no hernia present, hydrocoeles below the age of 1 year usually resolve spontaneously. Primary hydrocoeles may develop in adulthood, particularly in the elderly, particularly common in hot countries, by slow accumulation of serous fluid, presumably caused by impaired reabsorption, which appears to be the explanation for most primary hydroceles although the reason remains obscure. A hydrocele can also be the result of a plugged inguinal lymphatic system caused by repeated, chronic infection of Wuchereria bancrofti or Brugia malayi, two mosquito-borne parasites of Africa and Southeast Asia, respectively. As such, the condition would be a part of more diffuse sequelae commonly referred to as elephantiasis, which also affects the lymphatic system in other parts of the body.