Doctors at the Mayo Clinic in Rochester, Minnesota, found that of four patients who started drinking green tea or taking green tea extracts, three showed clear improvements in their condition in the following months.

The patients all had chronic lymphocytic leukemia, or CLL, a form of leukemia. Like all types of leukemia, CLL is a cancer of the blood and bone marrow, in which abnormal white blood cells replace healthy blood cells.

Reviewing the records of the patients, the doctors found that three showed signs of a regression in their cancer after they started to drink green tea or take green tea capsules. The fourth had an improvement in her white blood cell count, though her disease remained unchanged by standard criteria.

In one case, the patient had been showing progressive swelling in her lymph nodes - one of the characteristics of CLL - before she starting taking green tea capsules twice a day. Over the next year, her lymph nodes steadily decreased in size, according to findings published online by the journal Leukemia Research.

Another patient showed an improvement in her white blood cell count after she started drinking eight cups of green tea per day.

These cases alone cannot prove that green tea or its extracts conferred the benefits, Shanafelt told.

A number of previous studies have suggested that green tea and extracts of the beverage have cancer-fighting abilities, possibly due to the tea's concentration of certain antioxidants - compounds that help ward off cell damage that can lead to cancer, heart disease and other ills.

EGCG is thought to be the most potent of these tea antioxidants. The Mayo study from last year suggested that EGCG might induce leukemia cells to self-destruct by interfering with the communication signals they need to survive. But the exact mechanism by which green tea may fight cancer remains unclear.