The popularity of herbal medicines is growing steadily all over the world, and a large number of people are using these products for self-medication. Although these medicines are being widely used, and the number of available products is increasing exponentially, most of these compounds have not been properly evaluated for potential toxic or adverse effects. It is on this background that the National Toxicology Program has performed comprehensive 2-year rodent studies to evaluate the toxicity and safety of popular herbal medicines, including Aloe vera, ginkgo, ginseng, goldenseal, kava kava, milk thistle, and turmeric oleoresin. Here we review the pathological findings in rodents, with special emphasis on the carcinogenic potential of these compounds, which included liver tumor responses (goldenseal, ginkgo, kava), intestinal tumor response (aloe vera whole leaf non-decolorized extract) and thyroid tumor response (ginkgo). The results of these studies together with information gathered from additional clinical trials from other NIH institutes would provide a more complete evaluation of the risk and benefits from herbal medicine use.