Healthcare giant Capio is behind the mistake, which was discovered last Friday.
Most of the affected patients are listed at one of the company's health centers in the Västra Götaland region.
Stefan Bremberg, chief physician and data protection representative at Capio local health care, tells the newspaper that the fault lies with the company's external supplier.
-We take this very seriously and want to know how this could happen.
We have not received a complete report from the supplier yet.
As soon as we were informed, we also initiated an internal investigation with us.
The incident has been reported to the Swedish Data Inspectorate.