Middle ear fluids (MEE) and matched sera (S) were obtained from 50 patients with serous otitis media and magnesium levels were measured to determine if magnesium concentration was distinctly varied in otitis media with effusion (OME). The MEE/S ratio was considerably raised along with transient sensory hearing loss in chronic OME when compared with acute OME. The higher magnesium level found in the MEE implies that it is probably produced locally by the middle ear mucosa and may contribute to the hearing loss found. We also regard the MEE/S ratio as a prognostic factor in OME.