Narrative:

Our final altitude for cruise was to be FL340. During climb out from ZZZ at approximately FL315; we heard the cabin warning horn. We immediately looked up to see the cabin pressure at 11000 ft. We donned our oxygen masks and referenced the QRH. The first officer was flying and we started a descent per the checklist and declared an emergency with ATC. The flight attendants were notified to don their oxygen as well. As our descent continued; we were able to keep the cabin below 14000 ft and eventually got it below 10000 ft while passing through approximately 18000 ft. We elected to divert to ZZZ1 since it was the nearest airport in my estimation. We performed an overweight landing (approximately 118000 pounds). Upon arrival at ZZZ1 we had contract maintenance address our problems. Maintenance control; dispatch; and myself decided to fly the airplane unpressurized back to ZZZ below 10000 ft. Mels for the pressurization and four pobs used along with an overweight landing inspection were performed. The flight was full with 137 plus 2. We had another carrier's pilot in the jumpseat. In our discussions following the event; all three pilots had not noticed anything abnormal during the climb check and again at FL180. In retrospect; I should have considered ZZZ2; but we felt we were closer to ZZZ1. My primary concern was trying to get the cabin under control and safely conduct the flight.

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Original NASA ASRS Text

Title: B737-300 EXPERIENCES LOSS OF PRESSURIZATION; THEN DIVERTS AND LANDS OVERWT AFTER DECLARING AN EMER AND MAKING AN EMER DSCNT.

Narrative: OUR FINAL ALT FOR CRUISE WAS TO BE FL340. DURING CLB OUT FROM ZZZ AT APPROX FL315; WE HEARD THE CABIN WARNING HORN. WE IMMEDIATELY LOOKED UP TO SEE THE CABIN PRESSURE AT 11000 FT. WE DONNED OUR OXYGEN MASKS AND REFERENCED THE QRH. THE FO WAS FLYING AND WE STARTED A DSCNT PER THE CHKLIST AND DECLARED AN EMER WITH ATC. THE FLT ATTENDANTS WERE NOTIFIED TO DON THEIR OXYGEN AS WELL. AS OUR DSCNT CONTINUED; WE WERE ABLE TO KEEP THE CABIN BELOW 14000 FT AND EVENTUALLY GOT IT BELOW 10000 FT WHILE PASSING THROUGH APPROX 18000 FT. WE ELECTED TO DIVERT TO ZZZ1 SINCE IT WAS THE NEAREST ARPT IN MY ESTIMATION. WE PERFORMED AN OVERWT LNDG (APPROX 118000 LBS). UPON ARR AT ZZZ1 WE HAD CONTRACT MAINT ADDRESS OUR PROBLEMS. MAINT CTL; DISPATCH; AND MYSELF DECIDED TO FLY THE AIRPLANE UNPRESSURIZED BACK TO ZZZ BELOW 10000 FT. MELS FOR THE PRESSURIZATION AND FOUR POBS USED ALONG WITH AN OVERWT LNDG INSPECTION WERE PERFORMED. THE FLIGHT WAS FULL WITH 137 PLUS 2. WE HAD ANOTHER CARRIER'S PLT IN THE JUMPSEAT. IN OUR DISCUSSIONS FOLLOWING THE EVENT; ALL THREE PLTS HAD NOT NOTICED ANYTHING ABNORMAL DURING THE CLB CHK AND AGAIN AT FL180. IN RETROSPECT; I SHOULD HAVE CONSIDERED ZZZ2; BUT WE FELT WE WERE CLOSER TO ZZZ1. MY PRIMARY CONCERN WAS TRYING TO GET THE CABIN UNDER CTL AND SAFELY CONDUCT THE FLT.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.