ACR BEGAN CLIMB IN RESPONSE TO CLRNC MEANT FOR ANOTHER ACR ACFT. POTENTIAL CONFLICT; BUT ERROR NOTED BEFORE CRITICAL SEPARATION LOSS.

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I WAS FLYING AND THE F/O WAS DOING THE NON FLYING DUTIES. WE WERE DEPARTING O'HARE. AFTER LEVELING AT 5000' MSL; WE WERE CHANGED OVER TO DEP CTL. AFTER THE F/O MADE THE INITIAL CALL; THE CTLR SAID; 'ROGER; ACR X CLB TO 14000'.' THE F/O READ BACK THE CLRNC. THE CTLR DIDN'T DISPUTE THE READBACK AND I STARTED TO CLB. IMMEDIATELY THEREAFTER THE CTLR CLRED OUR COMPANY FLT Y; WITH A FLT NUMBER VERY SIMILAR TO OURS; TO 14000'. THE F/O SAID TO ME; 'WAIT; I THINK OUR CLB CLRNC WAS FOR SOMEONE ELSE.' I LEVELED MY CLB AT 5400' MSL. ALMOST IMMEDIATELY THEREAFTER; THE CTLR SAID TO US; 'ACR X; I HAVE YOU LEVEL AT 5000'.' THE CTLR THEN POINTED OUT INBND TFC. I QUICKLY DSNDED TO 5000' MSL. I'M NOT SURE WHETHER THIS CLRNC ERROR WAS CAUSED BY THE CTLR OR US. THE FREQ WAS VERY CONGESTED AND THE CTLR WAS VERY BUSY. THE CTLR MAY HAVE CONFUSED THE SIMILAR FLT NUMBERS AND ISSUED AN ERRONEOUS CLRNC. THE HEAVY WORKLOAD ALSO PROBABLY CAUSED THE CTLR TO FAIL TO COMMENT ON OUR READBACK. ON THE OTHER HAND; THE ERROR MAY HAVE BEEN CAUSED BY US TAKING ANOTHER FLT'S CLB CLRNC. THE F/O AND I ARE NOT SURE WHERE THE FAULT LIES. HOWEVER; WE FEEL THE ERROR WOULD NOT HAVE OCCURRED IF THE FREQ AND CTLR WERE NOT OVERLOADED.