Tympanometry measures the flow of sound energy in the external ear canal under conditions of changing ear-canal pressure. To evaluate this technic as a means of detecting middle-ear effusion, we studied acoustic susceptance and conductance in 91 children. We made the measurements using tones of 220 and 660 Hz over a pressure range of -300 to +300 mm of water. The mean value for susceptance at 660 Hz gave best results: 83 of 84 tympanograms of ears with middle-ear effusion had values less than 0.16 millimhos (mmho). Among 128 studies of normal ears, 113 had values greater than or equal to 0.16 mmho (P less than 0.001). We conclude that a mean acoustic susceptance at 660 Hz less than 0.16 mmho in a child four months of age or older indicates the presence of middle-ear effusion.