The chest x-rays revealed abnormal findings for 39 of the patients, five (12.8 percent) of whom experienced adverse outcomes, including death or the need for mechanical ventilation. For the other 58 patients, chest x-ray findings were normal, although two (3.4 percent) of the patients experienced adverse outcomes. The mean age of patients in the study, which included 53 men and 44 women, was 40.4 years.

"Abnormal findings in the periphery of both lungs and in multiple zones of the lungs were associated with poor clinical outcomes," Dr. Aviram said.

Although a normal chest x-ray did not exclude the possibility of an adverse outcome, Dr. Aviram said the study's findings can help physicians better identify high-risk H1N1 patients who require close monitoring.

"In H1N1, as in various types of community-acquired pneumonia, initial chest x-rays may not show abnormalities that develop later in the course of the disease," Dr. Aviram explained. "Further x-rays should be performed according to the patient's clinical course."