The Welsh Ambulance Service said 11 of its vehicles queued up outside Wales' largest hospital because there were no beds available for patients.
Patients waited for an hour outside The University Hospital of Wales
Some 999-patients had to wait for an hour outside the University Hospital of Wales (UHW) in Cardiff during the early hours of Thursday.
It followed an emergency admissions crisis at another hospital this week.
The situation had returned to normal by 0700 GMT, a spokesman for the ambulance service told the BBC.
The situation began on Wednesday evening and at one point there were no available ambulances to send to emergency calls.
It is the second time in a month that ambulances have had to queue outside the UHW because of a high volume of 999-calls and a shortage of available beds.
In a statement, a spokesman for Cardiff and Vale NHS Trust said: "The emergency unit at the UHW was under particularly heavy pressure last night.
"The trust was in contact with neighbouring hospitals and the picture at Cardiff was reflected across most major hospitals in the south east Wales area.
"Due to the fact that the UHW was, effectively full, with no beds available, patients had to wait in the emergency unit and that had a knock-on effect on the ambulance service.
"Its crews experienced delays as they waited to discharge their patients to the emergency unit.
"Every effort was made to discharge as many patients as possible from the hospital to make room for new patients but, unfortunately, demand exceeded bed availability."
Less than two weeks ago the Welsh Ambulance Service declared a state of "special emergency" after record demand in Cardiff and the Vale in mid-February.
People were urged to dial 999 only in life-threatening cases after calls reached "unprecedented levels".
Speaking about the situation, the chairman of the British Medical Association's Welsh Council said there would always be "peak and troughs" within the system.
Dr Tony Calland said: "It is difficult to predict these sudden surges. It is a capacity problem, you've got difficulty getting people out, mainly because there are a lot more elderly people in hospital."
He added the aging population was putting huge pressure on social services to find carers for them once they leave hospital.
"There are constraints on the nursing homes - many have had to close because they are uneconomical.
"There is pressure on social services. There is pressure all along the line."
Some patients' had to be transferred to nearby Llandough hospital.
On Monday people waited more than two hours on to be admitted to the Royal Glamorgan Hospital in Llantrisant as emergencies "overwhelmed" the hospital.
Up to seven ambulances queued outside the hospital.