The use of tubes and catheters in clinical therapeutics and diagnostics is growing rapidly. The distal ends of these devices are placed in a variety of tubelike body cavities, and in many cases reside there for extended periods to provide therapy or monitor physiologic function. Sonic techniques to guide the placement, monitor the position, and determine the patency of such tubes or catheters are under development. With a miniature microphone and speaker attached to the proximal end of the tube, a pulse--echo approach is used to gather patency and position information from reflections that arise from within the tube and body cavity, respectively. Design parameters such as the input pulse bandwidth can be optimized based upon the tube and body cavity geometry, including wall properties and branching. This approach has been used to monitor breathing tubes in neonates, and allows placement to within 4 mm of the desired location and quantification of decreased lumen area to within 10% of the actual obstruction. Thus these sonic systems offer the potential to reliably and inexpensively monitor tubes and catheters in the clinical environment.