Antonio Silveira left his residence at 3:40 p.m. Saturday on foot from his home on the 1100 block of Mersey Avenue (Washington Manor Area).
Silveira is 67 years old and at-risk due to not taking his prescribed medication.
He suffers from memory loss and diabetes.
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He was last seen wearing a gray hat, black jacket, blue pants and black shoes.
"Please, if someone sees him. Let us know," said Suhey Silveira, his wife. "We're really worried. This is going to be the second night he will be out."
Anyone with information about Silveira's whereabouts can call San Leandro police at (510) 577-2740.
KGO-TV staff contributed to this report
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