Ted Szal was tracked down in Beaverton where he told investigators he had no idea anyone was looking for him since he disappeared from the Chicago area in the 1970s.
"I believe Christmas has come early for the Szal family," said Cook County Sheriff Tom Dart. "Being able to tell an 88-year-old father that his son, whose picture he has been carrying around for 34 years in his breast pocket, has been found alive is something special."
Szal, 59, had been presumed dead at the hands of Gacy after he vanished in 1977. His car was found parked at O'Hare International Airport but no one ever heard another word from him until this week.
The Chicago Sun Times said Szal told investigators he had skipped town when he was 24 years old due to a dispute with his family and the breakup of his marriage. His fate became an open question recently when a DNA test revealed remains believed to be his were actually someone else. He was traced to Oregon through his Social Security number.
Cook County authorities this year launched a new push to identify all 33 of Gacy's victims.