Generally, the present invention is directed to a light-guided catheter for direct visualization of placement through the skin. An embodiment of the invention includes a method for transcutaneous viewing and guiding of intracorporeal catheters into a body that comprises inserting a catheter into the body having at least one lumen and internally illuminating the catheter with light capable of propagating through the blood and tissue to an external viewer outside of the body. The illumination may be point source, continuous, single or multi wavelength. A method of treatment of blood conditions is also disclosed using direct placement of a photo treatment source in the blood stream.