An endoscopic laser instrument is adapted to treat selected areas of body tissue, such as the treatment and evaporation of tumors occurring on the inner of a bladder. The instrument comprises an outer tubular sheath having an optical fiber extending therethrough to emit a laser beam onto an adjustable mirror that reflects the laser beam exiting the fiber to selected areas of the bladder without changing the character of divergence of the beam once it has emerged from the fiber. A standard endoscope viewing optic is mounted in the sheath to permit a surgeon to visually inspect the bladder through a wide visual field. A gas, such as carbon dioxide, is continuously circulated through the instrument and into the bladder for inflation and purging purposes.