Antitrust Improvements Act of 2003 - Amends the Sherman Act to increase criminal penalties for restraint of trade among the States or with foreign nations, monopolizing trade, or specified other restraints of trade among U.S. territories, States, foreign nations, and the District of Columbia.

Directs the U.S. Sentencing Commission to review and amend its guidelines and policy statements to: (1) ensure that they reflect the serious nature of the offenses and penalties, the growing incidence of serious antitrust criminal offenses, and the need to modify the guidelines and statements to deter, prevent, and punish such offenses; (2) consider whether the guideline offense levels and enhancements for Sherman Act violations are sufficient to deter and punish and are adequate in view of the increases in penalties, and whether the guideline offense levels and enhancements for such violations are consistent with recent amendments applicable to white collar offenses; (3) ensure reasonable consistency with other relevant directives and with other sentencing guidelines; (4) account for any additional aggravating or mitigating circumstances that might justify exceptions to the generally applicable sentencing ranges; (5) make any necessary conforming changes to the sentencing guidelines; and (6) ensure that the guidelines adequately meet the purposes of sentencing.

Repeals provisions that prohibit importing articles from a foreign country into the United States systematically below market value with the intent of destroying or injuring a U.S.industry, and authorize private lawsuits by injured persons for treble damages.