Hydrocele of the tunica vaginalis testis has been conventionally used as an absolute indicator of filarial disease in most clinical surveys. The prevalence of filarial etiology in 100 consecutive hydroceles was studied using clinical, parasitological, histopathological and immunological parameters. Filarial etiology could be proved in 57% of hydrocele cases using major criteria: presence of microfilaria in hydrocele fluid, presence of chyle in hydrocele fluid, demonstration of adult worm in tunica, ratio of fluid antibody titer to serum antibody titer more than 2 and presence of filarial antigen in hydrocele fluid. The results of other tests in these 57 cases were used to define the minor criteria. In the other 43 cases, based on the minor criteria, 12 hydroceles could be classified as likely to be due to filariasis and the rest were probably non-filarial. Thus only 69% of hydroceles were definitely or probably filarial.