The performance of shock wave (SW) lithotripters is often assessed by measuring the breakage efficiency of model stones. Such in vitro tests are usually performed by positioning the stones in plastic mesh baskets. We asked if the plastic mesh affects the properties of SWs, and whether this alteration influences cavitation fields and stone breakage efficiency. Experiments were conducted using a research electrohydraulic lithotripter patterned after the Dornier HM3 (HM3-clone), and a clinical electromagnetic lithotripter (XX-ES). Shock waves were measured at the geometric focus (F2) of the HM3-clone lithotripter using a fiber-optic probe hydrophone (FOPH-500). A 2.5×4mm extruded polypropylene mesh was positioned ×8mm or ×40mm prefocally. Cavitation bubbles were recorded using a multiframe high-speed camera (Imacon 468). Breakage of gypsum cement stones was assessed using the XX-ES lithotripter. SWs generated by this lithotripter propagate downward, so that when stones were positioned in an open-top basket, SWs could reach the stone without passing through the mesh that held the stone. This was compared to a mesh basket in the form of a cylinder, with which SWs had to pass through the mesh to reach the stone. Acoustic measurements showed that the mesh had little effect on the leading positive-pressure phase of the SW, but attenuated to some degree the tensile component of the pulse. Proximity of the mesh to F2 affected the waveforms such that the mesh positioned at 40mm prefocally was almost acoustically transparent, but the effect was much more pronounced when the mesh was within several millimeters of F2. High-speed camera images showed that cavitation was reduced beyond the mesh. Stone breakage was less efficient when SWs had to pass through the mesh to reach the stone. When SWs had to pass through the mesh, the number of SWs to break stones was 634*125 SWs, which was greater (P<0.002) than the 448±44 SWs needed to break stones in the open-top basket. These findings show that mesh material-generally considered to be acoustically "invisible"-may have a measureable effect on lithotripter SWs, cavitation fields, and stone breakage efficiency. These factors should be kept in mind when interpreting in vitro test results, particularly in studies assessing the performance of SW lithotripters or in studies on the mechanisms of SW action.