Social Security and Medicare Lock-box Act of 2003

Social Security and Medicare Lock-box Act of 2003 - Amends the Congressional Budget Act of 1974 to make it out of order in the House of Representatives or the Senate, until there is both a social security and Medicare solvency certification by the Boards of Trustees of the social security and Federal Hospital Insurance Trust Funds, respectively, to consider: (1) any concurrent budget resolution that would use any portion of the baseline budget surpluses; or (2) any legislation or amendment if the enactment of such legislation or amendment would use any portion of such surpluses.

Defines "baseline budget surplus" as the sum of the on- and off-budget surpluses contained in the most recent baseline budget projections by the Congressional Budget Office at the beginning of the annual budget cycle and no later than the month of March.

Establishes conditions under which a portion of such surpluses is determined to be used in the case of legislation affecting revenues or direct spending or an appropriations bill.