The Department of Health suggests that up to 350,000 patients could be on a waiting list to see a consultant at an out-patient department.
Recent figures up to March indicate that nearly 160,000 could be on a waiting list.
The department said that not all of the country's hospitals returned figures for that out-patient waiting list report.
The Special Delivery Unit said today that it estimates the total national outpatient waiting list once validated is likely to be between 325,000 and 350,000 patients.
It says this needs to be seen in the context of a national service that provides around 200,000 new and follow-up outpatient consultations a month.
The main problem suggested was that some patients have to wait an inordinate amount of time for an outpatient consultation.
Recently the HSE confirmed that over 800 patients were waiting four years or more to be seen at an outpatient clinic.
The total waiting list for assessment or treatment at a public hospital in March was put at around 218,000 patients, with not all hospitals having reported data.