The Guardian applied under the Freedom of Information Act, asking the Department of Health to send the financial returns submitted by 507 NHS organisations across England at the end of the first half of the current financial year.

The department sent a DVD packed with electronic files showing how the trusts performed between April and September - and what their forecasts were for the year as a whole.

The file for each trust contained a dozen or more spreadsheets, each with thousands of items of information. The Guardian wrote software to extract some of the interesting bits.

This helped us to identify the trusts with the biggest cumulative deficits. It also showed the actual performance of every hospital, ambulance service and mental health trust in England. This information is disclosed for the first time on Guardian Unlimited today.

The first column of our table shows the surplus or deficit made by each trust for the six months to the end of September.

Column two shows the surplus or deficit as a percentage of turnover.

Column three shows the trust's forecast for what the surplus or deficit will be by the end of the financial year in March.

Column four shows that as a percentage of its forecast annual turnover.

Column five shows the impact this would have on the trust's cumulative surplus or deficit - rolling the forecast results for 2006/7 into the surpluses or deficits that already lay on its books.

Column six expresses that as a percentage of 2006/7 turnover. This is a somewhat rough and ready indicator of the size of the trust's difficulty.

Trusts have a statutory duty to balance their accounts over a run of years and a high percentage in column six suggests a big problem.