There is a lot mentioned of how grape seed extract has shown to cause cancer cells to die. There is strong evidence to support that grape seed extract is effective against cancer.

A team of scientists from the US and China have discovered that grape-seed extract kills laboratory leukemia cells by making them commit suicide, thus showing the potential value of natural compounds in the treatment of cancer. Grape Seed Extract Kills Cancer Cells In Lab, (last visited Aug. 25, 2010).

That sounds pretty promising and I encourage you to follow the link as there is more good information about this grape seed extract study. It talks about more details of the study like what kinds of cancers were tested.

The University of Maryland's web site comes through too with some information about what is known about research related to grape seed extract for cancer.

Studies have found that grape seed extracts may prevent the growth of breast, stomach, colon, prostate, and lung cancer cells in test tubes. However, there is no clear evidence yet whether it works in humans. Antioxidants, such as those found in grape seed extract, are thought to reduce the risk of developing cancer. Grape seed extract may also help prevent damage to human liver cells caused by chemotherapy medications. Talk to your doctor or pharmacist before combining antioxidants with any chemotherapy drugs to make sure they interact safely together. Grape seed, (last visited Aug. 25, 2010).

That sounds good, but as mentioned, this refers to studies done in test tubes. Studies done on humans can and often do produce different results.

As for how grape seed may fight cancer, Wikipedia sheds light on a possible mechanism:

A polyphenol contained in grape seeds is resveratrol which may interfere with cancer cell growth and proliferation, as well as induce apoptosis, among a variety of potential chemopreventive effects.[3][4] Wikipedia, Grape seed extract, (as of Aug. 25, 2010).

But, the promising research does not suggest grape seed is certainly a cure:

"With these results, we are not suggesting that people run out and buy and use grape seed extract. That could be dangerous since so little is known about doses and side effects," said Rajesh Agarwal, Ph.D., professor in the Department of Pharmaceutical Sciences at the University of Colorado Health Sciences Center in Denver.

Still we should hope that more research is done with grape seed and cancer. I think the results of all of the grape seed extract research I've read about look incredibly promising.