Cystic fibrosis sufferers campaigning for a new drug to be made available on the NHS are to gather at Holyrood today.
The treatment Orkambi could give patients an almost normal life expectancy if they start taking it when they are young enough.
But while a deal was reached in May to make it available in the Republic of Ireland, the drug has not yet been approved for use by the NHS in any part of the UK.
The Cystic Fibrosis Trust said that of the 10,800 people in the UK with the disease 3,296 could benefit from the drug - including more than 240 patients in Scotland
James Barrow, head of external affairs at the charity, said: ''Cystic fibrosis is a cruel and life-shortening condition that places a huge daily burden on people living with the disease.
''Orkambi is a major step forward in the treatment of cystic fibrosis, which could help people live longer and has the potential to save the lives of thousands in the UK.
''The drug deals with the underlying cause of cystic fibrosis and prevents irreversible lung damage, which is especially important for children as they stand to benefit from it the most.
''If a deal between the Government and the manufacturer over the price of Orkambi is not reached soon, we fear people in desperate need of the drug will run out of time.''
As well as the protest at Holyrood, similar demonstrations will be held outside Downing Street and at the Welsh and Northern Ireland Assemblies.
Cystic fibrosis sufferer Robert Barker, from Carstairs in Lanark, has undergone an "amazing" transformation after being treated with Orkambi, which he has been taking since January.
Prior to that his wife Claire said she was "contemplating life without him" as he spent much of his time in hospital and was on oxygen 24 hours a day.
But now Mr Barker is able to enjoy cycling with his six-year-old daughter Katy as Mrs Barker said: "Orkambi has given me and my daughter back her Daddy.
"After taking the drug for a short time Robert felt improvements in himself. He gradually came off his day time oxygen and further moving to bring off his night oxygen. Robert can get about normally without being completely puffed out. He can enjoy spending time with his family and doing normal family things.
"Robert is back at work and his weight is up over 64kg. He has spent time landscaping our garden and working on our house. People take for granted nipping to the local shop for milk but this was something Robert couldn't manage before.
"The transformation is amazing."