A £10,000 reward has been offered for information relating to the death of Jay Abatan.
He died after being assaulted on a night out in Brighton in January 1999.
Detectives said they want to hear from anyone "who has so far felt unable to speak out."
The 42-year-old was assaulted while waiting for a taxi outside a nightclub with his brother Michael.
The father-of-two hit his head on the pavement and suffered a fractured skull which led to swelling and bleeding in his brain.
He died in hospital five days later.
You may not appreciate the importance of what you saw or heard so we urge you to report it, you may hold the piece of the puzzle that allows us to build the picture to provide answers for the family of Jay."
Officers are now offering a £10,000 reward for any information that leads to the arrest and conviction of someone responsible for the death of Mr Abatan.
Two men were arrested and charged in connection with his death, but these charges were later dropped.
They were also cleared of affray and assault during a trial.