Strokes cost the nation at least $30 billion a year, but the bills can range from $11,000 to $70,000 a patient and don't always correlate with survival, a new study says.

"Why the disparity? That's the $64,000 question," study author Dr. David Matcher of Duke University said Wednesday. "We are now trying to understand the relationship between what we do, what it costs and what it accomplishes."Strokes are the nation's third-leading killer after heart disease and cancer. They strike some 500,000 people a year, killing 150,000 and leaving others unable to move or even speak.

They are caused when a blood clot or fatty deposit blocks an artery or a blood vessel hemorrhages in the brain, killing brain cells. No approved treatments exist to stop a stroke; all doctors can do is help patients recuperate.

The National Stroke Association said Wednesday's study is the first to put a price tag on strokes - and found they cost more than all other illnesses except heart disease and cancer.

The study, commissioned by the federal Agency for Health Care Policy and Research and unveiled at a Washington neurosurgery conference, analyzed 1991 Medicare records of stroke victims. It concluded that medical care, rehabilitation and lost productivity cost $30 billion in the first three months - the time when problems could be positively attributed to stroke.

The average medical cost per patient was $15,000. Bills ranged from $11,000 to a high of $70,000 in some severe cases. One in 10 patients had bills of more than $35,000.

Nationally, 11 percent of patients died during their initial hospitalization. After three months, 20 percent had died. Forty-five percent left the hospital for a rehab center or nursing home.

Regionally, researchers discovered:

- Mortality was lowest in New England, 19.4 percent, and highest in the Mountain region from Montana to Utah, at 24.3 percent. But both regions spent an average of $13,700 per patient.

- Of stroke victims who live, 42 percent in the Central Northwest region (from North Dakota to Kansas) returned home, compared with 62 percent in the Central Southeast (Kentucky, Tennessee, Alabama and Mississippi).

- Regionally, states spent from an average low of $13,000 per patient to an average high of $17,000. Rural areas spent about $11,000 per patient, compared with $18,000 in cities.

Differences in stroke severity and overall health can't entirely explain the cost and survival disparities, so scientists now are looking at doctors' practices, Matchar said.

Stroke costs are one part of the five-year study. Doctors now will figure out which treatments offer victims the best chance of recovery at the best price.