A hospital trust is "satisfied" that it was not responsible for the breach that left confidential medical papers strewn in a Hertfordshire street.
Whipps Cross Hospital has also launched an investigation
Documents from St Bartholomew's Hospital, City of London were among those found last week in residents' gardens in Northaw, near Potters Bar.
But an internal investigation by Barts and The London NHS Trust found the breach did not happen at its sites.
The papers revealed serious illnesses suffered by patients.
A trust spokesman said: "Barts and The London is satisfied that this breach did not occur at any of its three hospital sites, nor through the Trust's confidential disposal procedures."
Correspondence from Whipps Cross Hospital in Whitechapel, east London, was also among the papers found by residents.
It too has launched an investigation into the incident.