The photosensitizing agent injected into the body is absorbed by all cells. The agent remains in or around tumor cells for a longer time than it does in normal tissue. When treated cancer cells are exposed to red light from a laser, the light is absorbed by the photosensitizing agent. This light absorption causes a chemical reaction that destroys the tumor cells. Light exposure must be carefully timed to coincide with the period when most of the agent has left healthy cells but still remains in cancer cells. There are several promising features of PDT: (1) Cancer cells can be selectively destroyed while most normal cells are spared, (2) the damaging effect of the photosensitizing agent occurs only when the substance is exposed to light, and (3) the side effects are relatively mild.