A steerable guidewire for percutaneous transluminal insertion into the vascular system and controlled negotiation of branches and turns therein, comprises an elongated tubular support which includes a proximal hypotube section and a distal spring coil. A movable pull wire extends through the support structure and is attached at its distal end to the rounded tip of the guidewire. A deflection section is provided in which a flexible deflection ribbon is mounted. The distal end of the deflection ribbon is attached to the pull wire and its proximal end is attached to the spring coil. When the pull wire is pulled axially, it applies a compressive force to the deflection ribbon which in turn applies a lateral component of force to the pull wire and spring coil in the deflection region. This causes the guidewire to bend with the deflection ribbon functioning as a fulcrum around which the guidewire bends.