Joseph Samperi, 67, was last seen in the 3000 block of Via Vista in Laguna Woods around 10 a.m. Saturday.
Samperi had a recent brain injury, causing memory loss, officials said. He has no personal identification other than a medical alert necklace.
He is in need of blood pressure medication.
Family members say Samperi lives in Pomona, but was visiting a relative in Laguna Woods and wandered away unnoticed from a medical transport vehicle.
OCSDPIO At risk missing: Joseph Samperi, 67yrs, area of Laguna Woods, on foot, needs medical care, has no ID, wearing jeans, sweatshirt. pic.twitter.com/GCafjCuwEh— OC Sheriff, CA (@OCSD) December 25, 2016
He was believed to be wearing blue jeans, a gray long-sleeve shirt and a dark-colored hooded sweatshirt.
Anyone with information was asked to call the Orange County Sheriff's Department at 714-647-7000 or 714-628-7170.