Since 1929, the Senate has held 53 secret sessions, generally for reasons of national security. Six of the most recent secret sessions, however, were held during the impeachment trial of President Bill Clinton. Four of those sessions were in February 1999 during the final impeachment deliberations. Two were in January 1999 to discuss
a motion to end the trial and another motion to call witnesses.1 In 1997, the Senate met in secret to consider the Chemical Weapons Convention Treaty, and in 1992, to debate the “most favored nation” status of China. The Senate also closed its doors during the
impeachment trial of a federal judge in 1933 and on six occasions while considering impeachment articles against three other federal judges in the 1980s.
http://www.senate.gov/reference/resources/pdf/98-718.pdf