'Lords of Dogtown' -- Original Z-Boy Goes Missing ... After Alzheimer's Diagnosis (UPDATE)

5:30 PM PT -- We just got word Ronnie was found in pretty bad shape. According to his family a stranger spotted him lying on the ground in the Silverlake area of L.A. -- at least a 25 minute drive from Culver City -- and took him to a hospital.

Ronnie was dehydrated, and is currently getting treatment. He's expected to be okay.

Ronnie Jay Leipold -- one of the original members of the legendary Z-Boy skating crew -- has gone missing ... and his family is extremely concerned because he suffers from Alzheimer's Disease.

62-year-old Ronnie -- who excelled as a pro surfer before joining the Z-Boy crew back in the '70s -- was last seen in Culver City wearing his medical ID bracelet.

Family members say Ronnie has no short term memory.

We're told the Culver City Police are actively looking for the skate legend -- and are hoping the public will aid in their search.

It's the second time Leipold has disappeared since being diagnosed with the disease 6 years ago.