Within the next few months, researchers at three medical centers expect to start the first test in patients of one of the most promising  and contentious  ideas about the cause and treatment of cancer.

The idea is to take aim at what some scientists say are cancerous stem cells  aberrant cells that maintain and propagate malignant tumors.

Although many scientists have assumed that cancer cells are immortal  that they divide and grow indefinitely  most can only divide a certain number of times before dying. The stem-cell hypothesis says that cancers themselves may not die because they are fed by cancerous stem cells, a small and particularly dangerous kind of cell that can renew by dividing even as it spews out more cells that form the bulk of a tumor. Worse, stem cells may be impervious to most standard cancer therapies.

Not everyone accepts the hypothesis of cancerous stem cells. Skeptics say proponents are so in love with the idea that they dismiss or ignore evidence against it. Dr. Scott E. Kern, for instance, a leading pancreatic cancer researcher at Johns Hopkins University, said the hypothesis was more akin to religion than to science.