Flight Crew Last 90 Days 100; Flight Crew Total 9000; Flight Crew Type 850
Situational Awareness
Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude
Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Environment - Non Weather Related; Procedure
I clicked off the autopilot and autothrottles and performed the descend RA down to 10;500 FT when the TCAS reported clear of conflict. The Captain; pilot not flying; told ATC that we were responding to an RA. Chicago Center cleared us to 10;000 FT near HIHRY. The general aviation aircraft was not talking to Center and was moving right to left on us.  We came within 200 FT altitude and lateral of the traffic. Center re-cleared us to 10;000 FT and we continued uneventfully.
Level at 11;000 on the TRTLL RNAV Arrival into O'Hare. ATC called traffic at 1 o'clock but before I could answer the radio we received an RA. First Officer flawlessly executed the RA Maneuver; disconnected the autopilot and autothrottles and descended 500 FT before the 'clear of conflict' announcement was made. I notified ATC that we were responding to an RA and that we had descended to 10;500 FT and had acquired the target visually. Airplane was a light Cessna and I estimate we were within 300 FT vertically and within 200 FT laterally of that traffic. ATC descended us to 10;000 FT. Autothrottles and autopilot were reengaged and the flight continued normally.
An air carrier crew reported taking evasive action during a TCAS RA with a Cessna on the ORD TRTLL1 RNAV near HIHRY intersection.