The paper describes compression tests on eight thin-walled cylinders of 3 ft. diameter and 0·035 in. wall thickness made of aluminium alloy plate. The lengths of the cylinders were either 6 ft. or 9 ft. Three of the cylinders were tested under axial compression up to buckling failure, and the initial buckling load, failing load and mode of buckling were observed. A further three cylinders were similarly tested, but these cylinders were subjected to internal pressure before applying the compressive load. The internal pressure had a twofold strengthening effect on these cylinders. Firstly, it induced a tensile pre-stress along the axis of the cylinder and, secondly, it resulted in the value of the compressive stress at which buckling occurred being greater than the buckling stress value for the unpressurised cylinders. The six cylinders were tested in a manner which allowed the end face of each cylinder to rotate about a diametral axis. A parallel platen device was used in testing the last two cylinders (in an unpressurised condition) which restrained rotation of the end faces of the cylinders. These tests enabled the effect of end restraint to be studied, and also enabled measurements of load-carrying capacity at large axial deflections to be made. The initial buckling loads, failing loads and modes of buckling observed in the tests were compared with existing large deflection theory.