Spontaneous tumor regressions are among the rarest and most mysterious events in medicine, with only several hundred cases in the literature that can be considered well documented. Regressions have most often been reported in melanoma and in kidney cancer. But the phenomenon may, in fact, be an everyday one, taking place beyond doctors' eyes. A recent study suggests that as many as 1 in 3 breast tumors may vanish on their own before being detected by a doctor. Why do some patients get lucky? Scientists are finding tantalizing evidence that the immune system, the body's defense against disease-causing microbes, kicks in to play a critical role in combating cancer. The evidence includes the fact that some unexplained remissions have occurred after infections, which may propel the immune system into high gear--possibly attacking the cancer tumor as well as the infection. The role of the immune system in controlling cancer has been hotly debated for decades--and indeed many scientists remain unconvinced. But Jedd D. Wolchok, an oncologist at New York's Memorial Sloan-Kettering Cancer Center, thinks there is a connection. A spontaneous remission, he says, is either divine intervention or the immune system." While few researchers directly study such cases--they are far too rare--they provide hints of what the immune system might be able to do if we could harness it.