Rep. Frank Ballance (search) resigned from Congress on Tuesday, saying a neuromuscular disorder has affected his ability to carry out his duties.

Ballance, 62, was diagnosed in February with myasthenia gravis (search), which causes muscle weakness. He said in a statement Tuesday that he was "no longer able to carry out the responsibilities of this office due to my current health condition."

The first-term Democrat, who is under investigation over the handling of funds at a foundation he started, had announced in May that he wouldn't seek a second term.

Ballance was in the hospital last week but has returned home, spokesman Ken Willis said.

"Over the weekend, he made the decision that because of his sickness he won't be able to fulfill his duty," Willis said in a telephone interview. "He's really sick."

Ballance is facing a joint federal-state investigation into activities the John A. Hyman Foundation (search), which ran a drug and alcohol counseling program in northeastern North Carolina.

The foundation has received $2.1 million in state money since 1994. An audit in October concluded it was riddled with conflicts of interest and made $325,000 in questionable payments.

Ballance, a longtime state legislator, was elected to Congress in 2002, and his term expires at the end at the year. State law requires the governor to call a special election if a congressman resigns in office.