Narrative:

While at cruise (FL400) we experienced cabin altitude warning. Both pilots donned oxygen and the captain declared an emergency and requested an immediate descent to lower altitude. Cabin altitude had reached 14000 ft. It did not begin to descend until we were below FL230. Once cabin was stabilized and safety of flight was assured; a temporary plan of action was formulated. The captain had a conversation with dispatch and maintenance to troubleshoot the problem; and notified cabin and passenger as time became available. Flight resulted in a safe uneventful landing at ZZZ. During our emergency descent; we remained within compliance of all ATC altitude restrs and instructions.

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Original NASA ASRS Text

Title: B757-200 CRUISING AT FL400 EXPERIENCES CABIN ALT WARNING. MAKES CTLED EMER DSCNT AND CONTINUES TO DEST.

Narrative: WHILE AT CRUISE (FL400) WE EXPERIENCED CABIN ALT WARNING. BOTH PLTS DONNED OXYGEN AND THE CAPT DECLARED AN EMER AND REQUESTED AN IMMEDIATE DSCNT TO LOWER ALT. CABIN ALT HAD REACHED 14000 FT. IT DID NOT BEGIN TO DSND UNTIL WE WERE BELOW FL230. ONCE CABIN WAS STABILIZED AND SAFETY OF FLT WAS ASSURED; A TEMPORARY PLAN OF ACTION WAS FORMULATED. THE CAPT HAD A CONVERSATION WITH DISPATCH AND MAINT TO TROUBLESHOOT THE PROB; AND NOTIFIED CABIN AND PAX AS TIME BECAME AVAILABLE. FLT RESULTED IN A SAFE UNEVENTFUL LNDG AT ZZZ. DURING OUR EMER DSCNT; WE REMAINED WITHIN COMPLIANCE OF ALL ATC ALT RESTRS AND INSTRUCTIONS.

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.