The way hospitals in the UK run outpatient clinic appointments is stuck "in the 18th Century", leading doctors say.
Every year millions of people travel to hospitals, where doctors check up on their health and discuss their care.
The Royal College of Physicians said many appointments were unnecessary - and outdated, inefficient systems meant large numbers were missed or cancelled.
It said the situation was frustrating for patients, and wasting money.
The RCP's report said the time had come to overhaul the system by embracing innovation: making more use of remote monitoring and telephone and video consultations.
Getting other staff, such as senior nurses, to run clinics closer to people's homes could also help.
It said a number of places had already started taking these steps, but much more needed to be done.
Dr Toby Hillman, of the RCP, said: "It's an 18th Century system. It should not be beyond us to tackle this."
What are outpatient appointments?
Outpatient departments are the busiest part of hospitals in terms of the numbers seen.
Last year there were 127 million appointments in the UK - nearly five times more than the numbers who came to A&E.
Patients come in to discuss their health as part of their ongoing care, or may see their doctor to discuss their recovery from surgery. or before they go under the knife.
The report pointed out these appointments can require a significant amount of time, cost and planning for patients because of things like travel, missing work and arranging childcare.
But in England one in five appointments is cancelled, or patients do not turn up.
Some of the cancellations are unavoidable, because of sickness or emergencies requiring staff to be deployed elsewhere.
But the RCP said the majority could be avoided if there were better systems in place.
How the system breaks down
There are a number of reasons why the service does not work smoothly.
Doctors surveyed - there were nearly 1,400 polled in compiling the report - said some appointments had to be cancelled because the results of tests were not yet available. or scans had not been booked in time.
One consultant described his experience of outpatient departments as "shocking", complaining of missing notes and results.
In cases where patients did not turn up, the report said there was evidence that they had tried to alert the hospital but there was not an efficient system in place to record the information.
The report said hospitals were also struggling to cope with demand - the number of appointments has doubled in the past decade.
This combination of inefficiency and sheer volume contributed to large numbers of appointments - 57% - running late.
How many appointments are unnecessary?
Doctors were asked if they felt a significant minority of outpatient appointments - between 10% and 20% - could be avoided.
One in four said this proportion of new patient appointments was simply not needed.
A similar number said this proportion of follow-up appointments could be avoided by using alternative methods, such as video consultations.
If the number of cancelled and missed appointments was reduced, this could also provide a significant saving.
But the RCP warned the system of funding hospital care needed to change.
Hospitals are paid per patient seen, so could find themselves penalised if they reformed the system.
NHS England medical director Prof Stephen Powis said doctors were just as frustrated as patients about the "antiquated" processes.
He said the issue would be tackled as part of the NHS long-term plan that is currently being developed so the outpatient service could be brought into the "21st century".
"The time has come to grasp this nettle," he added.