Tameside Hospital has been warned in a national report that its death rates could be 17 per cent higher than they should be according to indicators.
The Dr Foster Hospital Guide assessed the trust as the third worst performing in England for deaths up to 30 days after treatment.
It praised the NHS trust along with Trafford's for being among the best at responding to internet complaints.
The trust said other Dr Foster figures showed it had normal death rates.
The tenth annual report from the body introduced a new Summary Hospital-level Mortality Indicator (SHMI), which it said looked at a range of factors and examined deaths that occurred up to 30 days after treatment.
The report said: "We are featuring four measures of mortality in this year's hospital guide.
"All these measures should be used as 'red flags' or warning signs. They indicate that there is a risk that poor-quality care is leading to higher than expected mortality; they do not prove that this is happening.
"We only apply these analyses to NHS acute trusts."
'Sense of perspective'
Tameside Hospital Medical Director Tariq Mahmood said: "We welcome the Dr Foster report and take its findings very seriously.
"We are still analysing the differences between our Hospital Standardised Mortality Ratio (HSMR), which is well within the normal range, and the new Summary Hospital-level Mortality Indicator (SHMI).
"It is also important to stress that Tameside Hospital is well within the normal range for the three out of four mortality indicators - HSMR, deaths in low risk conditions and deaths after surgery - used by Dr Foster.
"By their very nature different statistical models can produce different results.
"Having worked very hard to understand and deliver clear improvements in our HSMR, we will also be working hard to determine the factors responsible for our SHMI and to address these both within the hospital and with our partner organisations."
"It is important to retain a sense of perspective when dealing with mortality indicators and the fact that there are several more under development underlines this," he added.
Wrightington, Wigan and Leigh NHS Foundation Trust was identified in the guide as being among trusts which have normal mortality rates during the week, but having higher than expected mortality rates for patients admitted at the weekend.
The Wigan-based trust was congratulated however on its treatment around knee and hip surgery where it performed above expectation.