Narrative:

Climbing through 10000 ft, flight attendant called to say there was a big air leak at the aft galley door. While talking to her, the 'cabin altitude' warning light came on. We immediately descended (with center approval) back to 10000 ft. We discussed what emergency checklist to use. The 'cabin altitude' warning directed us to the rapid depressurization checklist, which we felt didn't apply, as we hadn't had an rapid depressurization. I still read the clean-up items to see if they applied. Then we got busy coordinating our return to lax. The mechanic found the cabin altitude control valve in manual (full open). We believe it was missed on our 'before start' checklist.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN MD80 ON CLB AT 10000 FT DIVERTED DUE TO UNABLE TO PRESSURIZE THE CABIN CAUSED BY THE CABIN OUTFLOW VALVE LEFT IN MANUAL FULL OPEN.

Narrative: CLBING THROUGH 10000 FT, FLT ATTENDANT CALLED TO SAY THERE WAS A BIG AIR LEAK AT THE AFT GALLEY DOOR. WHILE TALKING TO HER, THE 'CABIN ALT' WARNING LIGHT CAME ON. WE IMMEDIATELY DSNDED (WITH CTR APPROVAL) BACK TO 10000 FT. WE DISCUSSED WHAT EMER CHKLIST TO USE. THE 'CABIN ALT' WARNING DIRECTED US TO THE RAPID DEPRESSURIZATION CHKLIST, WHICH WE FELT DIDN'T APPLY, AS WE HADN'T HAD AN RAPID DEPRESSURIZATION. I STILL READ THE CLEAN-UP ITEMS TO SEE IF THEY APPLIED. THEN WE GOT BUSY COORDINATING OUR RETURN TO LAX. THE MECH FOUND THE CABIN ALT CTL VALVE IN MANUAL (FULL OPEN). WE BELIEVE IT WAS MISSED ON OUR 'BEFORE START' CHKLIST.

Data retrieved from NASA's ASRS site as of July 2007 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.