Narrative:

Upon reaching cruise; we checked hydraulic quantity and noted the left system quantity was at 60% and decreasing. We conferred with dispatch and maintenance. Our weight was about 209000 pounds; so it was decided since the center and right hydraulic system were normal we should continue to ZZZ. All appropriate checklists and procedures were followed. We declared an emergency with ATC 30 mins out to give time for ground to set up. We did manual gear extension and altitude flap extension. Landed uneventfully. Stopped on runway. Fire crew checked aircraft; found hydraulic fluid leaking on right gear. We had gear pinned and towed to gate with fire trucks following as precaution.

Related reports:

Google
 

Original NASA ASRS Text

Title: AN MD80 EXPERIENCED A L HYD SYS FAILURE. THE CREW DECLARED AN EMER AND CONTINUED TO THE DEST ARPT FOR AN EMER LNDG.

Narrative: UPON REACHING CRUISE; WE CHKED HYD QUANTITY AND NOTED THE L SYS QUANTITY WAS AT 60% AND DECREASING. WE CONFERRED WITH DISPATCH AND MAINT. OUR WT WAS ABOUT 209000 LBS; SO IT WAS DECIDED SINCE THE CTR AND R HYD SYS WERE NORMAL WE SHOULD CONTINUE TO ZZZ. ALL APPROPRIATE CHKLISTS AND PROCS WERE FOLLOWED. WE DECLARED AN EMER WITH ATC 30 MINS OUT TO GIVE TIME FOR GND TO SET UP. WE DID MANUAL GEAR EXTENSION AND ALT FLAP EXTENSION. LANDED UNEVENTFULLY. STOPPED ON RWY. FIRE CREW CHKED ACFT; FOUND HYD FLUID LEAKING ON R GEAR. WE HAD GEAR PINNED AND TOWED TO GATE WITH FIRE TRUCKS FOLLOWING AS PRECAUTION.

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.