Multicellular tumor spheroids (MCTS) grown from the bladder cancer cell line RT112 and from the prostate cancer cell line PCA were exposed to 200 or 800 electromagnetically generated focused ultrasound shock waves. RT112 cells showed a distinct but transient decrease in proliferation whereas the effect of PCA cells was less pronounced. Flow-cytometric measurements of DNA content and Ki67 expression revealed no significant changes in the cell cycle distribution. The capacity of RT112 cells exposed to 800 shock waves to re-grow as MCTS was markedly decreased, indicating an alteration of intercellular adhesion.