The Healthcare Commission has launched an investigation into high mortality rates at Mid Staffordshire NHS Foundation Trust.
The regulator took the decision in response to data suggesting that the trust's rates of mortality are high, particularly in emergency admissions.
It said that it was concerned about the effectiveness of the trust's systems for monitoring mortality rates.
The Healthcare Commission's head of investigations Nigel Ellis said: 'An apparent high rate of mortality does not necessarily mean there are problems with safety…it does [however] require us to ask questions which is why we are carrying out investigations'.
The investigation will examine the ability of the trust's information systems to provide an accurate picture of mortality rates and identify potential problems 'so that swift action can taken, wherever improvements need to be made'.
The regulator will also examine the quality of care at the trust, in particular that given to older people, following a number of complaints by patients.
Mid Staffordshire trust chief executive Martin Yeates said: 'I would like to repeat the Healthcare Commission's statement that if our services were unsafe they would have taken action already'.