It is generally assumed that the gas enclosed in the middle ear and mastoid air cell system is subject to continuous absorption, and that partial pressures for oxygen and carbon dioxide equal those of "mixed venous blood." We previously determined the concentrations in 10 adults with healthy ears by means of mass spectrometry. The results showed that the value for oxygen was in the expected range, but that the value for carbon dioxide was higher than in "mixed venous blood." The aim of the present investigation was to measure the gas concentrations with the same technique in diseased ears, to see if there are any differences in middle ear gases between healthy and diseased ears. In terms of group means, the present measurements in 13 ears of children with otitis media with effusion or atelectasis did not differ from the values previously obtained for healthy ears of adults for any of the gases.