From CNN:

The Senate Ethics Committee is investigating Sen. John Ensign, and allegations of improper conduct stemming from an affair with the wife of a former aide. “Whenever allegations of improper conduct are brought to the attention of the Senate Ethics Committee, we open a preliminary inquiry,” committee spokeswoman Natalie Ravitz told CNN. But, in keeping with the committee’s tradition of secrecy, Ravitz went on to say that the “ethics committee doesn’t comment on ongoing investigations.” Because of the tight lipped nature of the ethics panel, it is unclear exactly what the Ethics Committee is probing. The Nevada Republican admitted in June to having an affair with Cynthia Hampton, the wife of former aide and long time friend Doug Hampton, who left Ensign’s staff in 2008 in the wake of that affair.