The wafer-thin, 5'51/2" actress, now 41, was long suspected of suffering from an eating disorder when she starred on the Boston-lawyer show, which aired on Fox from 1997 to 2002.
While she doesn't now admit to a specific disorder, she confesses that she was undernourished when she worked on the show.
"At the time of all that, I was seriously stressed," she said in an interview with the London Daily Mirror.
"I was working 15-hour days on the set and then I was dealing with the end of the show, which was basically my life," the actress said.
"I started under-eating, over-exercising, pushing myself too hard and [abusing] my immune system. I guess I just didn't find the time to eat. I am much more healthy these days," she told the newspaper.
In the days when "Ally McBeal" was one of the most popular shows on TV, her skeletal appearance alarmed viewers.