Health Care Antitrust Improvements Act of 2003

Applies the rule of reason standard to negotiations between a health plan and two or more physicians.

Awards attorneys' fees to a substantially prevailing plaintiff in certain actions only when the defendant's conduct was unreasonable or in bad faith.

Authorizes health care cooperative ventures negotiating with a health plan to file notification with the Attorney General, thereby limiting any potential recovery from conduct within the scope of such notification to actual damages (not punitive or treble).

Prohibits tying arrangements (linking the participation in one product line to participation in another) between a health plan and health care professional, except as specified.

Directs the Attorney General to establish: (1) at least six demonstration projects where health care professionals in project site States may act together to jointly negotiate contracts and agreements with health plans to provide health care items and services for which benefits are provided under such health plans; and (2) an Advisory Committee on Health Plan Negotiations.

Excludes from this Act any negotiations or agreements between health care professionals and health plans pertaining to the provision of benefits under Federal programs, including Medicare, Medicaid, SCHIP (State children's health insurance program), uniformed services' or veterans' medical care, Federal employees' health benefits, and Indian health care.