The objectives of this phase I/II study were: i) to determine whether electrochemotherapy (intralesional bleomycin + electric pulses) could be effective in companion animals with different, large neoplasms compared to chemotherapy (conventional intralesional bleomycin); ii) to identify potential toxicities; iii) to preliminarily assess the electric field requirements. Twenty-two patients received intralesional bleomycin + administration of permeabilizing electric pulses. Specifically, after the injection of the drug, sequences of 8 biphasic electric pulses lasting 50 + 50 micros each, with 1 ms interpulse intervals, were delivered in bursts of 1300 V/cm for cutaneous and soft tissue lesions, and of 800 V/cm for oral mucosal and exposed soft tissue neoplasms, using caliper electrodes. The treatment was well tolerated and side effects were infrequent. Nevertheless, two previously unreported toxicities (drug-induced vasculitis and pulmonary thromboembolism) have been identified. A high response rate (complete remission + partial remission: > 80%), often long lasting (> 40%) was obtained. Furthermore, results of this trial were compared to a subset of veterinary cancer patients treated with bleomycin single agent, observing a remarkable superiority of the combined treatment (p < 0.01). Altogether, results suggest that electrochemotherapy is a potentially advantageous rescue protocol for bulky, even relapsing neoplasms of companion animals. Further investigations in this field might allow developing improved protocols for the treatment of down-staged relapsing cancer in pets as well as in humans.