Existing diagnostic methods for otitis media with effusion are inadequate. We designed, built, and tested an acoustic reflectometer to overcome such inadequacies. The probe is placed at the entrance to the external auditory canal, whereupon a swept tone (1800 to 7000 Hz) is generated. The device records sound amplitude representing the sum of incident and reflected sound. This sum reaches a nadir at a frequency for which the quarter wave length corresponds to the distance from the microphone to the tympanic membrane; at this frequency reflected sound is maximally out of phase with incident sound. We measured this nadir (in decibels) and correlated the decrease in sound level at the nadir with the presence or absence of middle ear effusion. With a diagnosis confirmed by acoustic admittance and pneumatic otoscopy (n = 290), and using a breakpoint of 4.0 dB, the sensitivity was 94.4% and the specificity was 79.2%. Acoustic reflectometry can be portable, results are virtually instantaneous, and the method is reliable independent of age, crying, cerumen, and lack of cooperation from the child.