The permeabilising effects of electric pulses on cell membranes and the use of ultrasound energy of various intensities, for both thermal effects and enhancement of drug and gene delivery, have led to extensive research into the potential applications of these systems in the development of novel anti-cancer treatments. In the present study we have demonstrated for the first time that the application of brief electric pulses `sensitises' tumour cells to the effects of low intensity ultrasound. The studies were conducted in human tumours established in athymic nude mice and in many instances resulted in the reduction of tumour mass. The combined electric field and ultrasound approach (CEFUS) was applied in vivo to a murine colon adenocarcinoma (C26) and a human oesophageal adenocarcinoma (OE19). The experiments performed demonstrated the anti-tumour effects of the combined therapy. Varying the electrosensitisation parameters used (voltage, waveform, electrode type) contributed to optimise the procedure. Exponential electric pulses with a peak of 1000 V/cm were initially used, but square wave pulses (1000 V/cm, 1ms, x2, 1Hz) were found to be just as effective. All ultrasound application parameters were kept constant during the study. The growth rate of C26 tumours treated with CEFUS was significantly reduced with respect to untreated controls at day 7 (96% of average initial tumour volume in CEFUS group versus 615% for controls, P < 0.05). Similar reduction was observed in OE19 tumours treated with CEFUS by day 4 (82% versus 232%, P < 0.032). Our preliminary data suggest that this novel technology could potentially be of wide application in clinical practice for the treatment of solid tumours and is worth further investigation. (c) 2005 Elsevier Ltd. All rights reserved.