UNABLE TO PRESSURIZE THEIR B757 AFTER TKOF FROM ORD; FLT CREW DECLARES EMER AND RETURNS TO DEP STATION. EXTERNAL AIR CONDITIONING DOOR FOUND MISSING.

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ON CLBOUT; WE NOTICED NO AIRFLOW INTO THE COCKPIT (GASPERS OR OVERHEAD). ALL INDICATIONS WERE NORMAL. LOUD RUMBLE WAS HEARD BELOW CAPT'S KIT BAG AREA FOR A FEW MINS ON CLBOUT. LOOKING INTO THE PROB (COCKPIT GETTING WARMER; CABIN NORMAL); WE NOTICED THAT AIR WAS BEING SUCKED INTO THE GASPERS. AROUND 21000 FT; WE NOTICED THAT THE CABIN WAS CLBING AT A HIGHER THAN NORMAL RATE AND LEVELED AT 23000 FT. WE REALIZED THAT THE CABIN WAS NOT GOING TO STOP THE CLB AS IT NEARED 10000 FT. WE ELECTED TO DSND. I DECIDED TO DECLARE AN EMER AND START THE DSCNT IMMEDIATELY. MASKS IN THE CABIN DID NOT DEPLOY. WE LEVELED OUT AT 10000 FT AND LOOKED AT OUR OPTIONS. WE TURNED BACK TO ORD AND ENTERED HOLDING S OF CGT TO MAKE SURE WE HAD NOT MISSED ANYTHING. ALSO; WE KNEW WE WOULD BE MAKING AN OVERWT LNDG AND WANTED TO BE SURE WE HAD COVERED THE CHKLIST FOR THAT. THE REASON WE DID NOT BURN DOWN TO 198000 LBS IS THAT IT WAS 87 DEGS IN THE COCKPIT AND WE FELT IT WAS BEST TO GET ON THE GND. LANDED ON RWY 32L (LONGEST RWY AND INTO THE WIND) WITH THE TRUCKS STANDING BY JUST IN CASE. ON GATE ARR; ONE OF THE EXTERNAL AIR DOORS WAS NOTICED MISSING.