A device to monitor the position and patency of neonatal breathing tubes (endotracheal tubes, ETT) was developed based on a pulse-echo technique originally employed in a system for adult breathing tubes [J. P. Mansfield and G. R. Wodicka, J. Sound Vib. 188(2), 167--188 (1995)]. A 100 (mu)s duration sound pulse is emitted into the neonatal ETT via a wave tube containing a miniature sound source and a miniature microphone. The microphone detects echoes from acoustic impedance changes arising largely from changes in cross-sectional area within the ETT lumen and airways. The delays and amplitudes of echoes from obstructions within the ETT are used to estimate their respective locations and sizes. An echo arising from the ETT tip provides an estimate of the cross-sectional area just beyond the tube tip and is used to distinguish between proper tracheal and erroneous bronchial or esophageal intubations. Determination of tracheal position is accomplished by tracking the delay of a characteristic echo which arises from the airways. During intubation in five anesthetized rabbits, the device tracked changes in ETT position with an accuracy of 1(plus or minus)1.7 mm (mean(plus or minus)s.d.) over 188 measurements, which is adequate for use in even the smallest infant.