Current design codes and standards provide little information on the flextural behaviour of circular concrete filled tubes (CFT) as there have been few experimental studies. There are significant differences in d/t-limits recommended in various codes for CFT under bending. This paper presents an experimental investigation of the flexural behaviour of circular CFT subjected to large deformation pure bending where d/t = 12 to 110. The paper compares the behaviour of empty and void-filled, cold-formed circular hollow sections under pure plastic bending. It was found that for the range of d/t≤40, void filling prevented local buckling for very large rotations, whereas multiple plastic ripples formed in the inelastic range for specimens with 74<d/t≤1 10. In general, void filling of the steel tube enhances strength, ductility and energy absorption especially for thinner sections. Based on the measured material properties, the plastic d/t-limit was found to be 112. A simplified formula is provided to determine the ultimate flexural capacity of CFT. The existing design rules for the ultimate moment capacity of CFT may be extended conservatively to a new slenderness range of 100<α s < 188.