People living in the last two remaining occupied houses in a mainly derelict terrace in Blackburn had to be cleared after an arson attack.
Four of a row of ten terraced houses on Coleridge Street were "substantially damaged" in the deliberate blaze at 0050BST, the fire service said.
Fire crews used two jets and two hose reels to stop the flames spreading from derelict houses to the occupied ones.
A joint fire and police investigation is underway.
A Lancashire Fire and Rescue Service spokesman said that it believed local youths, who had put themselves in "considerable danger", were responsible for starting the fire.
The occupants later returned to their homes.