It is often difficult to evaluate reports of ultrasonically induced in vitro biological effects with respect to their clinical significance. An in vitro effect can be regarded as a real biological effect. However, acoustic exposures^1,2 and predominant physical and biological interactions and mechanisms involved in an in vitro effect may not pertain to the in vivo situation. Results from in vitro experiments suggest new end points and serve as a basis for design of in vivo experiments. In vitro studies provide the capability to control experimental variables that may not be controllable in vivo and thus offer a means to explore and evaluate specific mechanisms and test hypotheses. Although they may have limited applicability to in vivo biological effects, such studies can disclose fundamental cellular or extracellular effects of ultrasound. Although it is valid for authors to place their results in context and to suggest further relevant investigations, extrapolations to clinical practice should be viewed with caution.