A three-year study of 57,000 stroke victims has found troubling evidence that, despite widespread awareness campaigns, many people experiencing symptoms of stroke do not act quickly enough to avert damage and disability.

In a report released Friday by the Centers for Disease Control and Prevention, a national registry of patients who suffered a stroke between 2005 and 2007 has found that nearly 40 percent used private transportation to get to a hospital emergency room rather than calling 911.

Bad decision, because as every neurologist will tell you: Time is brain.

Only about 20 percent go to a hospital within two hours of the onset of stroke symptoms — which include weakness or numbness on one side of the body, sudden vision problems, confusion and difficulty in speaking or understanding or a sudden bout of extreme dizziness or severe headache with no known cause. That delay is particularly important because the treatment most effective in reducing death and disability from strokes caused by a blockage of a blood vessel — tissue plasminogen activator (or tPA) — must be administered no later than three hours after the onset of symptoms to be effective. Other stroke treatments also drive down death and disability, but are most effective when given promptly.

(A study published last month may offer some reprieve from that timetable. The study, published in the British journal Lancet, suggested the window within which tPA could be administered might safely be expanded to 4 1/2 hours. But if you are experiencing stroke symptoms, don't count on it — call 911 immediately.)

Getting to the hospital in time, apparently, is no assurance that tPA will be administered. Just under 40 percent of patients who fit the criteria for getting the clot-busting drug actually received it during the study period, which tracked patients from Georgia, Massachusetts, Illinois and North Carolina. Many hospitals and physicians do not have access to tPA or the expertise to administer it, and the drug's fearsome risk — of bleeding in the brain, particularly if it is administered after the prescribed window has closed — discourages many from using it.

Each year, 795,000 people in the United States suffer a stroke, and for 610,000 of them it is their first. You don't want to be one of them. Quit smoking, maintain a normal weight, take your blood pressure medications. If you or a loved one could be a victim, learn the signs. And if you see them, don't dally: Call 911.