In an electrohydraulic lithotripter (device to treat kidney stones), an underwater spark at the near focus of a rigid ellipsoidal reflector produces acoustic cavitation at the far focus. A reflector made of pressure release foam, however, dramatically reduces cavitation with little change in acoustic pressure amplitude (∼ +35 MPa, ∼−10 MPa). The pressure waveform at the focus of a rigid reflector is a positive spike followed by a negative tail. The negative tail sets bubbles into a growth-and-collapse cycle hundreds of ms long. With a pressure release reflector, a negative phase precedes a positive spike. Bubble growth induced by the negative phase is quickly (<10μs) snuffed by the spike, and collapse is weak. Cavitation was measured by (1) listening for bubble collapse and (2) recording bubble-induced pits on aluminum foil placed along the reflector axis. Pit size was used as an experimental measure of cavitation intensity. Intensity was reduced 3–10 times by the pressure release reflector. Results are in agreement with calculations from the Gilmore equation. Comparing the bioeffects of the two reflectors provides a way to assess cavitation’s role in lithotripsy. [Work supported by ONR, ARL:UT IR & D, and NIH.]