Chiropractic has never been shown to cause stroke, though it has also never been definitively ruled out either.  The plain fact is that it is so rare that it is nearly impossible to study enough to prove causality.  However there have been recent studies that indicate that chiropractic may lower your risk for stroke by decreasing hypertension.  “This procedure [Chiropractic manipulation of the Atlas] has the effect of not one, but two blood-pressure medications given in combination,” study leader George Bakris, MD, told WebMD. “And it seems to be adverse-event free. We saw no side effects and no problems,” adds Bakris, director of the University of Chicago hypertension center. Full Article Here

There is a group out there the “Chiropractic Stroke Awareness Group LLC” who claim to make people aware of the risks of chiropractic.  To clarify there are risks associated with chiropractic treatment.  Those risks are less than the risk you take by taking a dose of Asprin, but they do exist.  According to recent studies the risk of stroke following a chiropractic adjustment (not because of, but following) is 1 in 5.85 million.  For perspective here is a comparative list:

Proven Possible Side Effects of…

Here are some recent articles on the topic:

Stroke risk not raised by chiropractic treatment – January 25, 2008 – BC Local News

Chiropractic care and stroke are not connected – Posted 1 February 2008 – By Dr. Stuart Kinsinger