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CHICAGO – A review of three and one-half decades of clinical cardiology trials shows that while an increasing number of women are being included in clinical trials, their numbers are still so low in some areas of research that it's questionable whether study conclusions can be legitimately applied to women.

Investigators from the Duke Clinical Research Institute reported the finding today at the annual meeting of the American College of Cardiology.

The researchers, led by Dr. Chiara Melloni, a research cardiologist at Duke, examined the numbers of women included in 156 randomized clinical trials cited by the 2007 AHA Guidelines for Cardiovascular Disease Prevention in Women.

They found that overall, women comprised 30.6 percent of the total number of participants enrolled, with the number growing significantly during the past 36 years.

In 1970, women comprised only 9 percent of those registered in the prevention trials. In 2006, that figure rose to 42.4 percent, although researchers say the latter figure reflects a striking increase in the number of single-sex trials.

The researchers found that location of the trials appeared to play a role in participation: More women were enrolled in clinical trials in the U.S. (45 percent) compared to those abroad (26 percent).

Clinical conditions appeared to make a difference, too, with the highest number of women found in trials for hypertension (41 percent), diabetes (39 percent), and stroke (37 percent), and lowest for trials related to heart failure (29 percent), coronary artery disease (24 percent), and high cholesterol (17 percent).

The funding source for the studies did not seem to have any influence on the numbers of women involved. Women comprised about 30 percent of all participants enrolled in both government and privately-funded trials.

"It's heartening to note some gain in the numbers of women taking part in cardiovascular disease prevention trials, but we are still seeing substantial deficits in female representation in many areas of research," says Dr. Kristin Newby, a cardiologist at Duke and senior author of the study. "The results of this study tell us that efforts to change that picture are not robust enough to make a difference and that we still have a lot of work to do to ensure that we can generate evidence-based, sex-specific treatment recommendations when they are appropriate."