To determine the effects of exposure of endothelial cells to therapeutic ultrasound on leucocyte adhesion to these cells, confluent cultures of bovine aortic endothelial cells in tissue culture flasks and on collagen coated coverslips were exposed to a maximum of 1.6 W/cm2 1 MHz continuous ultrasound for 15 min, then incubated with neutrophil-rich suspensions containing a known number of viable human leucocytes for intervals from 1 to 240 min. After incubation, nonadherent cells were decanted, the number of leucocytes in the eluted suspension determined and the percentage of adhered cells calculated. Endothelial cell viability was not significantly reduced by ultrasound or by incubation with leucocytes. Whereas untreated endothelial cells were flattened and had surface microvilli, those exposed to ultrasound were plump, rounded and more widely separated along their borders. The percentage of leucocytes that adhered to sonicated endothelium was significantly higher, and this proportion increased with the duration of incubation. Thus, exposure of endothelial cells to ultrasound in vitro can cause intercellular separation and increase the adhesion to leucocytes. If such changes were also to occur in vivo, therapeutic ultrasound would potentiate the inflammatory response.