The trust that runs Watford General Hospital has been identified as one of 16 across the UK with higher than average mortality rates.
In the last financial year the number of deaths in Watford, St Albans and Hemel Hempstead hospitals also breached the national upper limit.
In particular the number of patients dying after breaking a hip was 50 per cent higher than the national average.
As a result, an "immediate review" was conducted by West Hertfordshire Hospitals NHS Trust to identify trends, and a significant number of measures were put in place.
These include investing £3.9 million a year to recruit 160 new nurses, increasing the number of consultants on wards - especially at weekends - and increasing the seniority of anaesthetists in complex surgery.
The trust’s mortality rate, or "hospital standardised mortality ratio" is calculated by dividing the number of deaths by the expected number and multiplying the figure by 100.
A risk rating of 100 represents the national average, and means that the number of patients who died was as the trust would expect.
The mortality rate for 2012/13 was 110.8, above the national average and taking it above the national upper control limit.
A monthly review group has been set up to track and review any variations in the figures.
Dr Mike Van Der Watt, medical director, said: "It is important to note that whilst our rate rose above the upper confidence level in 2012/13, it remains within ‘expected levels’ when viewed over the last three years.
"In addition, our data shows that we significantly under-reported the number of patients who were treated on a palliative care basis. This would have increased our mortality rate.
"As a result, we have reviewed the way that we record patients who are receiving palliative care to ensure we do not inadvertently skew our overall mortality levels."
The increase is one of four "elevated risks" identified by the Care Quality Commission in a monitoring report.
Within these figures was a higher-than-expected mortality rate relating to trauma and orthopaedic conditions and procedures.
These include head of femur and hip replacement, craniotomy for trauma, reduction of fracture of bone and shunting for hydrocephalus.
A report by the hospital trust identified mortality rates for patients with a broken femoral neck, commonly known as a broken hip, were 50 percent higher than the national average.
Dr Van Der Watt, Medical Director said the trust treats roughly 450 patients a year who have suffered a hip fracture.
He added: "We have set up a dedicated committee of specialist doctors and nurses to regularly review the care provided to these patients and to track our mortality rate as we move forward. "Initial findings show that our mortality rate for inpatients who have suffered a hip fracture has dropped in recent months. However, further analysis is needed."
Patients are now reviewed by a multi-disciplinary team before they undergo surgery, are anaesthetised and operated on by senior clinicians, given the appropriate time to recover after their operation before returning to their ward, and are seen by a specialist doctor in the days after.
Elderly patients will also be seen by a "care of the elderly consultant" at the earliest possible opportunity.
The CQC also drew attention to "serious concerns" from the General Medical Council regarding education, and an incident of whistle-blowing from within the trust.
The report is the first the hospitals’ inspectorate has produced about the trust, after it ditched its previous Quality and Risk Profiles.