Marshfield Clinic Health System: 35,952 individuals, Nov 2025.

OCR classifies the incident as a hacking or IT incident, with the breached information held in email. Hacking/IT Incident is the type recorded on 83 of the 126 Wisconsin filings (66%) and on 81% of all filings submitted in 2025. Email appears on 33% of Wisconsin filings.

No business associate is recorded on the filing; 31% of Wisconsin filings do involve one. At 35,952 individuals the breach is 9.8 times the Wisconsin median filing of 3,657 and 9.0 times the national median of 4,000 across all 7,884 filings. It sits in the 10,000 to 99,999 band, which holds 1,938 filings.

As of 28 August 2026 the case is listed by HHS OCR under Cases Currently Under Investigation. OCR has not published a closing summary, so this page is limited to the fields on the filing itself; the archive entry that follows a closed investigation usually adds a short account of what was exposed and what the entity did afterwards.

Wisconsin's breach notification statute (Wis. Stat. 134.98) requires notice to affected residents within 45 days after learning of the unauthorized acquisition. Its attorney general threshold: no AG notification requirement. The HIPAA Breach Notification Rule ran alongside it: notice to affected individuals and to HHS without unreasonable delay and no later than 60 days after discovery, which is the clock that put this filing on the portal.

Multiplying 35,952 individuals by IBM's $160 customer-PII cost per record (Cost of a Data Breach 2025, the figure the site's calculator uses) gives a modelled $5.75M; the 2026 edition's $192 gives $6.90M. Both are modelled estimates with the method shown, not costs disclosed by Marshfield Clinic Health System. For scale, IBM's 2026 average cost of a healthcare breach is $6.64M, so this filing models at 1.0 times the sector average.