The percentage of women partners and associates working at NLJ 250 law firms this year fell to its lowest point since 2006.

Women associates and partners accounted for 29.9% of all attorneys at NLJ 250 firms. Five years ago, women made up 32% of attorneys at those firms.

The findings are from this year’s NLJ 250, The National Law Journal‘s annual ranking of the nation’s largest law firms. The percentage of women was based on the average number of attorneys working at those firms.

The average size of NLJ 250 law firms this year was 505 lawyers, with women partners and associates comprising 151 of those attorneys. This year’s decline to 29.9% was the lowest since the NLJ began reporting gender breakdowns in 2006. Last year, women partners and associates made up 30% of NLJ 250 attorneys.

Since 2006, the percentage of women partners and associates has declined slightly each year. Those declines occurred even as firms got bigger in 2006, 2007 and 2008.

The decline for women partners and associates likely is due to a number of factors. Structural changes that law firms have implemented in recent years lowered their numbers, said Stephanie Scharf, president of the National Association of Women Lawyers Foundation. The increased use of staff attorneys and more lateral hiring also have reduced the percentage of women partners and associates. In addition, Scharf said, the latest recession, which hit associate ranks harder than partner ranks, thinned women’s numbers, since the percentage of women associates is higher than that of women partners.

“Those changes tend to impact women in a negative way more than men,” Scharf said.

Another reason is that practicing law is becoming less attractive to women, said Jessie Kornberg, executive director of Ms. JD, an online resource for women attorneys. “When there’s no improvement to point to and prominent women are talking about opting out, those are discouraging messages being sent to women,” Kornberg said.

Indeed, the number of women admitted to law schools in 2009 was up by 3.9%, but the number of men rose by 5.7%, according to the Law School Admission Council. In 2008, the number of women applicants fell by 0.1%, compared to an increase of 0.1% for men.

There are many success stories about women practitioners who love the profession, Kornberg said, but big law firms have not done a good job of using and retaining the talent that women offer.

“There starts to be very little reason to stick around,” she said.

Leigh Jones can be contacted at ljones@alm.com.