Legionnaire's Disease, now recognized as a puzzling and dangerous strain of penumonia, may contribute to the deaths of more than 70,000 Americans a year, according to a new study released yesterday.

To the surprise of doctors at an Ohio hospital who studied records of virtually every autopsy over a 12-month period, Legionnaire's bacteria were present in the lungs of eight patients. Some died of pneumonia. Some died of another disease with pneumonia as a contributing, and in two cases unsuspected, cause.

This means that the Legion organism -- now called Legionella pneumophilia -- figured in 3.9 percent of all the deaths of persons 15 and older, according to Dr. Dale Fay and associates of Riverside Methodist Hospital in Columbus, Ohio, and collaborating scientists at Ohio State University.

If the same incidence holds nationally, they say in this week's Journal of the American Medical Association, there may be 71,370 unrecoginized Legionella-associated deaths every year.

Also, they say, many such deaths may occur in persons who get the infection from organisms they encounter during long stays in the hospital. All eight infected persons studied probably developed the disease in Riverside Methodist Hospital, Fay and colleagues say.

The organisms may often come from air conditioning systems, these doctors add. Legionnaire's disease bacteria were found in the Columbus hospital's system and have been widely found in air conditioning systems generally.

Doctors at the federal Center for Disease Control in Atlanta say just 55,000 pneumonia deaths -- deaths with pneumonia as the prime cause -- are reported yearly.

They estimate that Legionnaire's disease causes approximately 25,000 pneumonia cases a year. Fifteen percent of them are fatal. The death rate can be reduced to 4 per cent when the disease is recognized in time and the right drug, usually erythromycin, is used.