The results fall short of an immediate cancer cure, but they are encouraging, medical professionals say.
JoEllen Welsh, a researcher with the State University of New York at Albany, has studied the effects of vitamin D for 25 years.
Part of her research involves taking human breast cancer cells and treating them with a potent form of vitamin D.
Within a few days, half the cancer cells shriveled up and died. Welsh said the vitamin has the same effect as a drug used for breast cancer treatment.
"What happens is that vitamin D enters the cells and triggers the cell death process," she told "Good Morning America." "It's similar to what we see when we treat cells with Tamoxifen," a drug used to treat breast cancer.
Tumors in Mice Disappear
The vitamin's effects were even more dramatic on breast cancer cells injected into mice.
After several weeks of treatment, the cancer tumors in the mice shrank by an average of more than 50 percent. Some tumors disappeared.
Similar results have been achieved on colon and prostate cancer tumors in mice.
People should take care not to read too much into laboratory studies, said Dr. Richard Besser, ABC News' senior health and medical editor. Positive effects in a petri dish or in rats may not necessarily mean similar results in humans, he said
It's also easier to treat cancer in mice than in people, Besser said.
Tips for Getting Adequate Amounts of Vitamin D
There is testing to determine whether patients are getting enough vitamin D, but Besser doesn't recommend it for everyone. If someone has repeated fractures or falls, his or her doctor may decide to do the test.
To ensure adequate vitamin D intake, Besser recommended that people check their diets and get enough sunlight.
Thirty minutes of sun twice per week -- generally between the hours of 10 a.m. and 3 p.m. -- should be sufficient, he said. He also said that such levels of exposure won't increase the risk for cancer.
If diet and sun exposure cannot be improved, people may also consider a supplement, he said.