In 100 children (150 ears) with chronic secretory otitis media the function of the Eustachian tube during treatment with grommet was investigated by air equalisation methods. Tubal function proved poor in the great majority at the beginning of the treatment, but towards its completion there was some improvement. After extrusion of the grommet, tubal function was investigated on the same material by tympanometry. 34% had normal middle-ear pressure initially, and 43% 12-18 months after closure of the perforation. There was no relation between tubal function shown by air equalisation methods and by tympanometry, and the air equalisation methods proved of less value than tympanometry in assessing the course and prognosis of secretory otitis. The pathogenetic theories - the ex vacuo and the secretory theory - are discussed in relation to the chronic tubal dysfunction found to be the most common direct cause of the disease.