The study, published on the Web site of the journal Health Affairs, said five of those conditions - heart disease, trauma, cancer, pulmonary conditions and mental disorders - accounted for 31 percent of the growth in health spending over that period.

The study ''is a focus on the patients who are driving the growth,'' said the lead author, Ken Thorpe, an Emory health policy expert. ''By focusing on a disease-based analysis of spending, we can compare the medical benefits we are purchasing.''

The cost of health care is an increasingly critical issue for families, employers and policy-makers. Health care is projected to account for 15 percent of gross domestic product in 2004, up from 11 percent in 1987. The cost of health insurance during the past three years has jumped by an average of 12.5 percent each year, the study noted. During the study's 13-year period, health spending rose at about double the inflation rate, Thorpe said.

The study examined health care spending by patients with the most expensive medical conditions and the factors that caused the increased spending.

One factor was substantial increase in the numbers of patients treated. For example, though the prevalence of mental disorders stayed basically stable between 1987 and 2000, the number of people treated for these conditions nearly doubled.

This surge was caused by both a rise in the identification and diagnosis of these disorders, as well as the increased availability of psychotropic medications, the study said.

Cases involving pulmonary disorders, such as asthma, rose 50 percent.

In eight of the top 15 medical conditions, the study found that a rise in the cost per case - rather than an increase in cases - accounted for most of the growth in spending.

The number of people treated for heart disease stayed about the same between 1987 and 2000. But the rise in the cost per treated case, caused mainly by more expensive and effective drugs and technologies, is what accounted for nearly 70 percent of the rise in medical spending for that category, the study said.

An increase in the prevalence of diseases such as diabetes and pulmonary disorders may be attributable to other factors, the study said. The growth of diabetes closely tracks the rise of obesity. And the rise of asthma has been linked to both in indoor and outdoor air pollution.

Health insurers and employers should seek to prevent the rise in such conditions as asthma, diabetes and hypertension, Thorpe said.

''Prevention has to be at the top of people's agenda to control health care costs,'' he said.

Costly conditions

The 15 most expensive medical conditions, in order, according to a new Emory University study:

* Heart disease

* Trauma

* Cancer

* Pulmonary conditions

* Mental disorders

* Hypertension

* Diabetes

* Arthritis

* Back problems

* Cerebrovascular disease such as stroke

* Pneumonia

* Skin disorders

* Endocrine disorders

* Infectious disease

* Kidney disease