Date: 201705
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Pressurization System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Critical; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Landed As Precaution
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Climbing through FL210 we received a Warning Horn (Intermittent) followed by Warning Light - Cabin Altitude. Analog Pressurization Controller Panel; Amber 'Auto Fail Light' illuminated. Cabin Altimeter/Differential Pressure Indicator showed Cabin at 3;500 FT and rapidly increasing.  No other Master Caution Lights; no DUAL BLEED; PACK TRIP OFF; WING-BODY OVERHEAT; or BLEED TRIP OFF lights. Simultaneously; [we advised ATC]; a turn to the left was made (off of the SID) and descent was initiated. The Crew O2 masks were put on and Crew communication was established; QRC/QRH checklists were performed.In the descent; with cabin stabilized at about 13;000 FT; Flight Attendants were briefed: 'STABLE airplane; rapid depressurization event; returning to [departure airport]. SECURE THE CABIN for arrival.' Same message passed to passengers level at 10;000 FT. All non-normal items were rechecked; complete. Also; word was passed to OPS via radio to inform of our return; asked OPS notify Dispatch; Maintenance Control; and Maintenance.Passengers O2 Masks did not auto deploy; as we did not exceed a cabin altitude of 14;000 FT. I made the decision to not manually deploy PAX O2 Masks. PM/First Officer agreed with the decision to not manually deploy PAX O2 Masks. Decision to not deploy passenger O2 Masks was made under Captain's Emergency Authority.   PM/First Officer handled all QRC and QRH Checklists and communicated with ATC.  (And; I must add; performed flawlessly all duties throughout the entire non-normal event.)  Reviews were made by PM of all non-normal checklists to ensure items were not missed and all checklists were properly complied with; outstanding work by this First Officer. Flight Attendants also performed with equal excellence and professionalism. Fire Department inspected aircraft clear of runway [and] we proceeded to [the] gate.  Support from Base; OPS; Ground; and Maintenance was phenomenal. OPS were under great pressure to handle our Passengers. 30 Passengers were rerouted and over 100 made the trip to [destination] with us in our replacement aircraft. I'm really proud of everyone and our Company.  We received terrific support from Operations; Chief Pilot; and Dispatch. Overall; a great day.


Second narrative: I think it all went fairly well. The training immediately kicked [in] and although it wasn't routine; we were able to run emergency procedures; execute a plan and follow it. I have to say; the Captain and the two jumpseaters did an excellent job.


Synopsis: B737 flight crew reported returning to departure airport after experiencing loss of cabin pressure.