I remember reading about a study in Madrid around 1999 or 2000 in which THC was found to kill brain tumors in mice. It was on Yahoo! News and the story was removed after a week or two, and I’ve never been able to find anything else about it… until now:

Dr Melamede said whereas nicotine activated carcinogenic compounds, THC - one of 60 cannabinoids derived from the cannabis plant - had been shown to inhibit them in mice cells.

“Compounds found in cannabis have been shown to kill numerous cancer types including lung, breast, prostate, leukaemia, lymphoma and skin cancer.”

But he said the effects of cannabis were complex as evidence also suggested low doses of THC could stimulate growth of lung cancer cells.

BBC.