More formal complaints were made to Lancashire Teaching Hospitals last year.
The Society for Acute Medicine says rising numbers of complaints across the NHS are unsurprising, warning that staff are "stretched across all sectors".
The Lancashire Teaching Hospitals NHS Trust received 598 written complaints from patients and their families in 2018, according to the latest NHS data.
That's an increase of nine per cent on the previous year, when 550 complaints were made.
Almost half the complaints raised issues about medical treatment received while in hospital, and a further 26 per cent were concerned about quality of care and support, including complaints about hospital food.
Trusts with limited funding can find themselves unable to resolve complaints, according to the Society for Acute Medicine.
The society's president, Dr Nick Scriven, said: "You feel for staff under incredible pressure, doing their best but knowing it won't please everyone.
"However, it is vital trusts engage openly with and seek to learn from the complaints process itself, and take appropriate action on complaints that are upheld."
At Lancashire Teaching Hospitals, staff resolved 589 formal complaints last year, including some from previous years.
In 28% of cases, evidence was found to support the complaint and an admission was made by the trust, and 40% were recorded as partially upheld.
The remainder were deemed to be unsubstantiated, frivolous or vexatious.
Patients' rights group Healthwatch said its members complain because they want to improve quality of care in the future.
Policy head Jacob Lant said: "Rather than just counting the number of complaints, what people want to see is what the NHS has changed as a result.
"This is the best way to build confidence in the complaints process, and show the public that the NHS is always willing to learn."
Acute trusts across England recorded 76,500 complaints last year, up from 57,000 in 2015.
NHS Improvement said this rise is in part because of improvements to the complaints process.