Abstract: A telescopic boom with a multistage, lockable hydraulic cylinder is provided with a protection against lateral buckling of the hydraulic cylinder, said hydraulic cylinder being supported by the more rigid boom members (telemembers). For this purpose, a multipart supporting device is provided, one part of said supporting device being secured to at least one unit of the multistage hydraulic cylinder and the other part thereof being secured to the inner wall of the at least one telemember. The two parts of the supporting device are continuously in engagement with one another so that the desired protection against buckling of the hydraulic cylinder will be guaranteed at all positions of the boom. Furthermore, suitable measures guarantee that, starting with the cylinder unit having the largest cross-section, it is always the cylinder unit having the next smaller cross-section which, in an orderly sequence, is first fully extended, the reverse order being observed during the retraction process.