Missed appointments are potentially costing hospitals more than £13 million a year.
Hospitals across Sussex have recorded more than 107,000 no-shows for outpatient appointments between April 2011 and the end of March.
“Did not attends” (DNAs) can have an impact on waiting lists as well as the financial cost of wasting staff time.
Health bosses believe someone not turning up can cost between £110 and £129 for each appointment.
Most trusts compensate by overbooking outpatient appointments to minimise the impact and staff with unexpected free time can fill it up by doing other jobs instead.
However, health bosses are urging people who know they are not going to turn up to let them know beforehand.
Some absences can be legitimate, with patients either not getting letters confirming their appointments on time or having to deal with an emergency.
However, many do not turn up because they had forgotten they had an appointment or did not need one any more but had not told anyone.
Brighton and Sussex University Hospitals NHS Trust recorded 45,208 missed appointments, but this is an improvement on the more than 50,000 the year before.
Western Sussex Hospitals NHS Trust had 30,875 no-shows, a slight rise on the year before, while East Sussex Healthcare NHS Trust had a small drop, to 31,793.
Work done at the Brighton trust to bring numbers down includes online cancellation forms and sending patients text messages.
Western Sussex Hospitals director of performance Adam Creeggan said: “Our trust has achieved one of the lowest rates of DNAs in the country, and is sustaining that strong performance.
“The biggest single reason for that success is that we no longer simply send patients a letter telling them when their first appointment is – instead we use a call centre approach to agree all first attendance appointment times with the patient. Getting that agreement makes it far less likely that people will not attend.
“However, we are still looking for ways to improve still further and continue to explore options such as text message reminders to make it easy for patients to attend.”
A spokeswoman for East Sussex Healthcare said: “Non-attendance at appointments wastes the time of doctors and nurses and causes inconvenience for other patients.
“We would urge patients to get in touch with us if they want to cancel their appointment.
“That way we can use the appointment for someone else and it will also mean that our waiting times are not affected.”
See the latest news headlines from The Argus:
- Tourist arrested on terrorism charge for taking a selfie video at Churchill Square
- i360 critic ends petition in shock U-turn over project
- Did those millions drop down the pothole? Council doesn't know how much taxpayers' money has been wasted
- Missing Dutch art is discovered in Sussex
- Brighton bucks the trend as city is among the the UK's highest for house prices