From minor accidents to deadly errors, medical mishaps occur every day in Connecticut's busy hospitals. But most are kept secret from the public under a five-year-old revision to the state's adverse-event reporting law, a Courant investigation has found.

The legislation was intended to compel hospitals to improve care and help patients assess the quality of the state's medical facilities. But hospitals balked after the reports were made public, and lobbyists persuaded the legislature to rewrite the law, narrowing the reporting requirements and adding a confidentiality provision, the Courant is reporting in a project by reporters Matthew Kauffman and Dave Altimari to be published Sunday.

Hospitals now disclose a fraction of the mishaps they once revealed. But even when the state is notified, the Department of Public Health keeps most reports under wraps. The details of more than a dozen sexual assaults are concealed in the health department's files, along with at least 30 cases in which sponges or other objects were left in patients' bodies after surgery, a Courant analysis found. Information on hundreds of serious falls is also kept under wraps by the department, as are the particulars of at least half a dozen cases in which newborns died or were seriously injured during childbirth.

While more than 900 reported adverse events are kept hidden by the health department under the revised law, cases that generate a full investigation are made public, and information on those events is contained in a searchable database below, and at www.courant.com/adverseevents.