East Sussex NHS Trust said the stick had been left by a member of staff near a trust building.
The information was not password protected.
The trust said it took data security "extremely seriously" and the loss was an "isolated incident".
Simon Keen, who was one of the patients whose data was on the stick, said: "They could have credit cards made, things could be purchased online - organised criminals pay a lot for this information.
"Yet the NHS is putting it on a computer stick and a member of the public is finding it in the street
"I find that really disturbing."
The chief executive of East Sussex Healthcare NHS Trust, Darren Grayson, said the data stick belonged to a member of staff and was not compliant with trust policy.
"It was an isolated incident and the trust takes the security of patient's personal information extremely seriously," he said.
More than 3,000 patients have been sent a letter of apology by the NHS after a computer memory stick containing their personal information was found by a member of the public.