Electrochemotherapy combines bleomycin and local electric pulses that allow cell permeabilization and free access of bleomycin to its intracellular target. We report the first veterinarian clinical trial of electrochemotherapy in 12 cats with spontaneous large soft-tissue sarcomas that suffered relapse after treatment with conventional therapies. Permeabilizing electric pulses were delivered using external surface electrodes, as well as new needle-shaped electrodes that were designed to be inserted in tumours for more effective treatment of several-centimetre-thick tumour nodules. The electric pulses were applied to the tumours several times from 4 to 15-30 min after a bolus intravenous injection of 0.5 mg kg(-1) bleomycin. Tolerance to treatment was excellent without general side-effects. The cats showed local inflammatory reactions for a few days and disease stabilization lasted from 2 weeks to 7 months. One partial regression was observed, and the general absence of nodule volume decrease can be explained by local fibrotic reactions. Histological analysis of biopsies also revealed massive tumour cell death. The cats' lifespan increased (P<<0.001), with a mean survival time of 6.1 months (maximum 18 months) compared with 0.8 months (maximum 1.5 months) for a group of 11 untreated control cats displaying similar carcinological features. Electrochemotherapy is clearly effective as a salvage treatment for large spontaneous solid tumours in adverse clinical situations and this is promising for future applications.