Volodymyr Shinkarenko, the head doctor at the hospital in Zaporizhzhia, was quoted by Ukrainian news media as saying Thursday that the fire broke out in medical equipment.
The patients were all on ventilators, but it wasn't clear if that was the equipment that caught on fire.
The patients' ages were 91, 76 and 64, and the doctor was 26. The cause of death for all is believed to be carbon monoxide poisoning.
Zaporizhzhia is about 450 kilometers (280 miles) southeast of the capital, Kyiv.