Being admitted to hospital with internal bleeding on a bank holiday pushes up the risk of dying by 41 per cent, say researchers.
Inadequate staff cover and a lack of access to hospital tests means patients are more likely to die than those admitted on normal week days.
Admission on a weekend also increases the risk of death by 13 per cent.
The research, by experts at the University of Swansea, looked at admissions for upper gastrointestinal bleeding, which can be caused by a range of conditions including ulcers and gastritis.
It confirms other studies which show a higher chance of dying if a person is admitted to hospital on weekends.
In June, a review found that the death rate among NHS emergency admissions across England increased by seven per cent at weekends in 2005/06.
That is the equivalent of 3,369 more deaths at the weekend than would normally be expected.
Experts said one reason was a lack of consultants and fewer specialist services available at the weekend, including diagnostic tests.
Last week, the Royal College of Physicians warned that patients are being left in the hands of junior doctors because of inadequate consultant cover on weekends.
Yesterday's study, published in the journal Alimentary Pharmacology and Therapeutics, analysed more than 24,000 admissions for upper gastrointestinal bleeding between 1999 and 2007 in Welsh hospitals, including emergency admissions.
Lead author Dr Stephen Roberts said higher death rates for weekend and public holiday admissions might result from to reduced staffing levels or delays in investigative procedures such as endoscopy (an internal examination).