Pacific Gas & Electric Co. spokesman Brian Swanson says staff found documentation on May 20 of a small leak on the same transmission line in 1988 about nine miles south of the neighborhood devastated by September's deadly pipeline blast.
Swanson says the company replaced a 12-foot portion of the leaking pipe the same year.
Eight people were killed and 38 homes destroyed in the inferno.
PG&E says staffers just found a report on the 1988 leak and turned it over to federal investigators.
A National Transportation Safety Board spokesperson says it's "troubling" that the utility only revealed the leak a few weeks ago, nearly nine months after the investigation began.
The NTSB hasn't determined the exact cause of the blast.
The Associated Press contributed to this report.