Abstract: A cystoscope of the invention comprises a main portion (10) and a tubular sheath (12, 110) that carries on its distal end a balloon (22, 114, 214) inflatable from the outer side of the cystoscope for spreading apart the prostate lobes in the prostatic part of the urethra and thus improving conditions for observation and operation on the prostate. The invention also provides a cystoscope convertable into a resectoscope which consists of a sheath (12, 110) and an outer sleeve (112), slidingly guided over the sheath. On the outer side of its distal portion, the sliding outer sleeve (112) carries an inflatable balloon (114) which is connected to an air-feed tube (118) located inside the sheath. The outer sleeve is connected to an inner sleeve (116) which slides inside the sheath. The distal end face (110b) of the sheath is inclined at an acute angle to accommodate the inclined optical portion of the cystoscope, whereas the distal end face (116a) of the inner sleeve is arranged substantially at 90.degree.