Beginning on 30 June 1980, the Cardiac Ward of the Hospital for Sick Children in Toronto experienced a marked increase in infant mortality following the division of the cardiology ward into two adjacent wards. Within two months, 20 patient deaths prompted a group of nurses to approach the unit's cardiologists, but the matter was kept in-house to avoid a "morale problem." The excess deaths continued through the following months.
The situation changed in March 1981. Metro Toronto coroner Dr. Paul Tepperman was called to the hospital on 12 March 1981 after the father of three-week-old Kevin Pacsai, who had died that day, showed unusual distress. Eight days later, on 20 March 1981, Tepperman learned of a January autopsy on Janice Estrella that had found an extremely elevated digoxin level in her bloodstream. He was subsequently told of another hospital autopsy that found thirteen times the normal concentration of the same drug in another deceased infant. The medication had not been kept under any security measures at the time. Police were called in and began searching staff lockers; another baby died from digoxin poisoning on 22 March 1981. Review of work logs, combined with colleagues' subjective impressions of one nurse's reactions to the deaths, led to that nurse's arrest and murder charge; she was released on bail.
The nurse charged, Susan Nelles, was acquitted at the preliminary hearing stage, and the case never proceeded to trial. The presiding judge found it not credible that more than one nurse could have been involved in a series of apparently motiveless murders, and Nelles could not have been responsible for a death that the judge determined was an additional murder but that had been excluded from her indictment. Data from the investigation, sent to the U.S. Centers for Disease Control and Prevention, identified nurse Phyllis Trayner as the only staff member on duty for all 29 deaths under examination. In September 1981, Trayner reported finding propranolol tablets in her food, and another nurse reported finding the tablets in her soup. Trayner denied any impropriety, was questioned in televised inquiry hearings, and resigned after the inquiry's report was published; she died in 2011.
A subsequent commission of inquiry listed eight of the baby deaths as murder and another 13 as highly suspicious, and a further death occurring after the inquiry began, apparently from digoxin poisoning, was not factored into its findings. The inquiry discounted claims by the hospital's own former chief of pediatrics that the deaths were not homicides and had not been proven to result from digoxin. In January 1982, unrelated illnesses among babies in a separate department were traced to epinephrine having been substituted for vitamin E. It has since been argued that a chemical compound capable of leaching from rubber medical tubing, and which laboratory tests can mistake for digoxin, may account for some of the deaths. No one was ultimately convicted in connection with the deaths.