The Hebridean Celtic Festival, which attracted about 1,000 local followers in its first year in 1996, now draws crowds of 15,000 annually to Stornoway, with half coming from outside the islands.
Over the years fans from over 100 countries have made the pilgrimage to the four-day festival – which this year is being held from 15-18 July.
They help double the population of the host town and provide the biggest money spinner for local businesses outside the festive season.
HebCelt organisers calculate the overall economic impact of the festival over the 20 years is up to £20 million at current prices, with bars, restaurants, cafes, accommodation providers and transport firms among those benefiting.
Since the first festival, 202,000 fans have attended concerts, with 10 per cent arriving from abroad, 40 per cent from other parts of the UK and the other 50 per cent made up of local followers.
Festival director Caroline Maclennan said: “As we look back over the 20 years, the growth of the festival has been phenomenal.
“It is now the biggest single annual event in the Outer Hebrides and is a vital economic generator with businesses saying the money they earn during festival week helps them until Christmas.
“We enjoy fantastic support from the local community and also from fans across the UK, Europe and beyond. We have regulars from the US and Australia and this year alone people have booked from Canada, all around Europe, Qatar and the United Arab Emirates.”
She added: “HebCelt really has come of age over the past two decades which is all thanks to the hard work and dedication of the local community, sponsors and support we get year after year from the public.”
The festival’s impact has been praised by local MSP Alasdair Allan.
He said: “The importance of HebCelt economically and culturally to the islands cannot be over-stated. It has been a huge draw in terms of tourism and in supporting and sustaining local businesses over 20 years, as the figures demonstrate.
“It does a wonderful job promoting traditional music and the Gaelic language and raising the profile of the islands to an international audience.”