Around 100 lawyers, 47 defendants and representatives of more than 1,000 civil parties in the case had assembled in the southern town of Taranto for the long-awaited opening of the trial.
But the first hearing lasted only a few minutes before it was adjourned to December 1st on the grounds that at least one defendant had not been given proper notice to appear.
Among the people on trial are a string of local politicians, including Nichi Vendola, the left-wing former governor of the region of Puglia, and two members of the Riva family which owns the site.
One of them, Fabio Riva, was extradited from Britain to face trial in June, having been effectively on the run since 2012.
Riva's billionaire father was placed under house arrest in 2012 but died last year.
The site in Taranto, which provides work for around 16,000 people, has been under special administration since 2013 after the Rivas were accused of failing to prevent toxic emissions including carcinogenic particles from spewing out across the town.
The plant used to churn out an estimated nine million tonnes of steel per year – about a third of the country's total production.
But experts found that the chemicals it emitted were behind high cancer rates and cardiovascular and respiratory diseases among workers and locals, who suffered from a “mortality excess” of between 10 and 15 percent.
Prime Minister Matteo Renzi has promised to clean up Ilva and sell it on but that currently looks a forlorn hope in light of the currently depressed state of a global steel industry blighted by chronic over capacity.
Despite the plant's status as one of the most polluting industrial sites in Europe, many locals want it to remain open for fear of the consequences of closure in an area with an already high unemployment level.