Firefighters spent almost eight hours tackling a blaze in a block of flats in Chesterfield.
Crews from Chesterfield, Staveley, Clowne and Clay Cross were called to St Helen's Street at about 8.45pm yesterday.
A Derbyshire fire service spokesman said: "The incident involved a fire in a block of flats.
"All occupants were out when crews arrived.
"It was most probably an accidental fire caused by smoking materials."
Firefighters left the scene at 4.30am today.