During the period from 1974 to 1981, surgery for acute mastoiditis was performed on 12 ears, giving an annual incidence of 0.004 per cent among cases of acute otitis media. All ears made a full long-term recovery. The low incidence is ascribed to the world-wide early use of antibiotics. During the same period 52 ears with secretory otitis media (SOM) underwent mastoidectomy. Histologically extensive mastoid inflammation was found in 96 per cent. The changes appeared as formation of secretory cells and cysts, resorption of bone, and infiltration of soft tissues with both mononuclear cells and polymorphonuclear leucocytes. The number of SOM patients undergoing mastoid operations account for 1.4 per cent of the patients admitted to hospital because of SOM. Thirty-four ears (65 per cent) have healed during the follow-up period (mean 2.9 years), while the tympanostomy tube is still in place in 18 ears (35 per cent). In the latter group, factors causing oedema in the pharyngeal end of the Eustachian tube are apparently still present.