A deflectable guidewire for a catheter includes a core extending from the proximal end of the guidewire toward the distal end thereof; a hollow sheath for encasing the core; the hollow core including a deflecting region at the distal end of the guidewire, the deflecting region having a first side and a second side, the first side being on an opposite side of a plane extending axially through the hollow means than the second side; the first side of the deflecting region being able to be compressed a greater distance than the second side of the deflecting region; the core being fixed to the hollow sheath at the distal side of the deflecting region; and a control handle mounted at the proximal end of the core and the hollow sheath for applying tension to the core with respect to the hollow sheath so that the tension causes the first side of the deflecting region to compress a greater amount than the second side, thus deflecting the deflecting region. In one embodiment, the core may include laser conducting optical fibers. Other features of the present invention include arranging the optical fibers in the catheter in such a way that bending stresses are distributed evenly throughout the fibers when the catheter is bent. One method of doing this is to arrange the fibers in rows or layers wherein each row or layer is twisted in a direction that is opposite to the direction of the adjacent rows or layers. It has also been found that flexibility is increased without unacceptable transmission losses by using optical fibers having a diameter of about fifty microns.