A telescopic crane boom including a plurality of telescopically assembled tubular box sections including a base box section and an innermost box section having an open cross-section, and a boom head with a frame fixed to the outside circumference of the innermost box section. The head has a plurality of deflecting pulleys mounted on the frame, but no part extending inside the open cross section on the innermost base box section, whereby at least one additional box section can be inserted into the innermost box section in order to extend the boom.