Narrative:

Upon leveloff at FL350 I felt the pressurization changing and noticed cabin altitude rate of climb/descent was varying +/-1000 FPM and the cabin altitude was slowly increasing. Thinking it might be the controller; I switched to alternate; but the variation continued. Referencing the QRH for rapid depressurization; I switched to manual but was unable to get the outflow valve to manually close. The cabin altitude reached 10000 ft; the altitude warning horn sounded; the pilots donned their oxygen masks; and I turned on the seatbelt sign. I declared an emergency and coordination a descent and turn back toward ZZZ. I referenced the QRH for emergency descent and let the flight attendants and passenger know we were dealing with a pressurization problem. I sent an ACARS message to the dispatcher notifying him of the situation. As we descended the cabin continued to climb; so I reselected 'automatic' to see if the controller could regain control of cabin pressure. The cabin altitude topped out at approximately 14000 ft and the cabin masks automatically deployed. Once we reached 10000 ft; the pilots discontinued use of oxygen masks and I let the flight attendants and passenger know they could also take their masks off. Return to ZZZ was uneventful and we terminated the emergency after landing; although I had notified ATC once we were below 10000 ft that special handling was not necessary.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B737 FLT CREW EXPERIENCED LOSS OF CABIN PRESSURE; DECLARED AN EMERGENCY; EXECUTED AN EMERGENCY DESCENT; AND RETURNED TO DEP AIRPORT.

Narrative: UPON LEVELOFF AT FL350 I FELT THE PRESSURIZATION CHANGING AND NOTICED CABIN ALT RATE OF CLB/DSCNT WAS VARYING +/-1000 FPM AND THE CABIN ALT WAS SLOWLY INCREASING. THINKING IT MIGHT BE THE CTLR; I SWITCHED TO ALTERNATE; BUT THE VARIATION CONTINUED. REFING THE QRH FOR RAPID DEPRESSURIZATION; I SWITCHED TO MANUAL BUT WAS UNABLE TO GET THE OUTFLOW VALVE TO MANUALLY CLOSE. THE CABIN ALT REACHED 10000 FT; THE ALT WARNING HORN SOUNDED; THE PLTS DONNED THEIR OXYGEN MASKS; AND I TURNED ON THE SEATBELT SIGN. I DECLARED AN EMER AND COORD A DSCNT AND TURN BACK TOWARD ZZZ. I REFED THE QRH FOR EMER DSCNT AND LET THE FLT ATTENDANTS AND PAX KNOW WE WERE DEALING WITH A PRESSURIZATION PROB. I SENT AN ACARS MESSAGE TO THE DISPATCHER NOTIFYING HIM OF THE SIT. AS WE DSNDED THE CABIN CONTINUED TO CLB; SO I RESELECTED 'AUTO' TO SEE IF THE CTLR COULD REGAIN CTL OF CABIN PRESSURE. THE CABIN ALT TOPPED OUT AT APPROX 14000 FT AND THE CABIN MASKS AUTOMATICALLY DEPLOYED. ONCE WE REACHED 10000 FT; THE PLTS DISCONTINUED USE OF OXYGEN MASKS AND I LET THE FLT ATTENDANTS AND PAX KNOW THEY COULD ALSO TAKE THEIR MASKS OFF. RETURN TO ZZZ WAS UNEVENTFUL AND WE TERMINATED THE EMER AFTER LNDG; ALTHOUGH I HAD NOTIFIED ATC ONCE WE WERE BELOW 10000 FT THAT SPECIAL HANDLING WAS NOT NECESSARY.

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.