Cold-formed steel C- and Z-section beams are widely used in light gauge steel building systems as flexural members. In their applications as flexural members, mono-symmetric C-sections and point-symmetric Z-sections are often subjected to transverse loads eccentric to the shear centre, leading to combined bending and torsion actions. However, research and design methods for cold-formed steel beams subjected to combined bending and torsion are limited. Hence this research has investigated the structural behaviour, strength and design of cold-formed steel C- and Z-section beams under the action of combined bending and torsion. Twenty-four tests were conducted on simply-supported beams subjected to a mid-span eccentric load. Two C-sections and two Z-sections were used in the tests with two different spans, each with three loading-eccentricities. A special test set-up was developed and used to simulate the different loading-eccentricities and to provide accurate boundary conditions. Numerical models of tested beams were then developed using ANSYS and nonlinear finite element analyses including the effects of large deformation and material yielding were performed. The numerical results agreed well with the test results in terms of ultimate strengths, failures modes and load-displacement curves. Finally the results from tests were compared with predictions from the current design equation for bending and torsion. This paper presents this investigation of cold-formed steel beams subjected to combined bending and torsion, and the results. •Cold-formed steel beams were tested to failure under varying levels of combined bending and torsion actions.•Numerical models of tested beams were developed to simulate their behaviour.•The results from tests were compared with predictions from the current design equation for bending and torsion.