Women held 13.6 percent of corporate director seats in 2003, up from 12.4 percent in 2001 and 9.5 percent in 1995, the study said.
The number of women board directors has increased at an annual average rate of 4.1 percent for the years 1995 to 2003. The study predicts that if the increase holds steady at that rate, women will comprise a quarter of all Fortune 500 company boardrooms in 20 years.
The number is likely to rise as the independence of boards has become increasingly important at a time when corporate governance issues are being fine-tuned following a wave of scandals, the nonprofit women's advocacy group said.
The search for more qualified outside candidates with fewer ties to chief executives and other directors will give more women a chance to serve on boards, the study predicted.
An increased demand for experts in fields like finance and human resources where women hold senior positions and vacancies arising because men are serving on fewer boards simultaneously, also should drive up the number of women board directors in the next few years, the study said.
Currently, 54 Fortune 500 companies have 25 percent or more women directors, up from 30 companies in 2001 and 11 in 1995. Even so, 54 companies have no women board directors and 208 have just one woman director. In 2001, 66 companies had no women in that position, down from 96 in 1995.
Women make up 46.5 percent of the U.S. work force and held a little more than 50 percent of managerial and professional specialty positions last year.
By Anusha Shrivastava