The head of the NHS in Wales says doctors are concerned about the "dramatic drop" in people seeking medical help in some areas, due to coronavirus.
There has been a 35% drop in emergency admissions to hospitals and a 25% fall in patients seeing their GP.
"Some people may be waiting too long to seek urgent assessment and treatment," said Andrew Goodall.
But he said it was important that this medical work continued.
Where has there been a fall?
-20%Ambulance arrivals at A&E
-35%emergency admissions in hospital
-20 to 25%patient activity at GP practices
What are the changes in NHS use in Wales?
- A&E attendances are down 60% compared with the same period last year
- Ambulance arrivals at A&E departments are around 20% lower
- There has been a 35% reduction in emergency admissions to hospitals
- GPs are working differently with fewer face-to-face contacts, but NHS Wales estimates that patient activity is down between 20 to 25%
Dr Goodall, NHS Wales chief executive, added: "Given the general drop in activity we have seen over recent weeks I am concerned that people may not be seeking medical help for illnesses and symptoms not related to Covid-19.
"Some people may be waiting too long to seek urgent assessment and treatment.
"We understand many services have been disrupted but we want to reinforce the message that urgent and emergency care services for physical and mental health are still open and accessible."
Roland Salmon, a retired public health expert, said: "We've heard from the Royal College of Midwives and from the Royal College of Paediatrics and Child Health, expressing concern that patients that they want to see aren't coming to see them.
"I know that oncologists and oncology patients find this a particularly difficult period because they have a very, very difficult choice," he told BBC Wales.
"Do they postpone treatment that they might need soon, or do they have the treatment and risk being immunocompromised at a point when the virus is still circulating?"
What's the advice?
- For parents if your child is unwell and you are concerned you should seek help
- Those with urgent dental pain should still call your dentist
- If you have a health complaint that is worrying you and will not go away you should call your GP
- If you or a family member are seriously ill or injured you should dial 999 or attend your nearest A&E department
Phone and video consultations are to be rolled out to community nurses, health visitors and community midwives and some clinics "very soon".
Meanwhile, in the daily update on the status of the NHS in Wales, Dr Goodall said about 46% of the acute hospital beds - around 3,135 - are currently empty.
There are 940 people with confirmed coronavirus in hospital and a further 395 suspected cases.
Critical care capacity was at 399 beds this week - with 49% vacant and available.