Diagnostic cardiac ultrasound are commonly assumed to pose no hazard to the patient-but this is not synonymous with being biologically inert. The production of intracellular reactive oxygen species (ROS) on endothelial cells is a key modulator of atheroprotective (at low level) and atherogenic (at high levels) actions. The aim of the study was to evaluate in vitro the effects on intracellular ROS of endothelial cells after ultrasound exposure of variable duration with commercially available cardiac imaging systems. Endothelial cells fluorescence was evaluated in vitro after sham (transducer off) exposure to ultrasound and after 5', 15' and 30' of ultrasound irradiation with second harmonic 1.3/2.6 MHz cardiac ultrasound scan (mechanical index 1.5). Intracellular ROS were 83 at baseline, and rose to 86, 112 and 122 fluorescence intensity at 1 h incubation after 5', 15' and 30' of ultrasound exposure respectively (P<0.01 for 30' versus baseline and 5' comparison). There were microscopic signs of endothelial damage only following 30' stage. Ultrasound exposure induced significant DNA laddering and LDH leakage after 15' of ultrasound exposure. Effects on endothelial cells could be reproduced by adding exposed extracellular medium to unexposed cells, and could be prevented removing exposed medium from cell culture or pretreating the medium with catalase. Cardiac ultrasound of current clinical diagnostic use increases intracellular oxidative stress on endothelial cells in vitro. This increase is accompanied by morphological evidence of endothelial damage only after longer exposure times, persists 1 h after withdrawal of ultrasound, and can be modulated over a wide range according to the duration of ultrasound exposure. Free radical production in the extracellular medium is the likely mediator of ultrasound effect.