The free vibration problem of unidirectional composite cylindrical helical springs is modelled theoretically as a continuous system considering the rotary inertia, shear and axial deformation effects. The first order shear deformation theory is employed in the mathematical model. The 12 scalar ordinary differential equations governing the free vibration behavior of cylindrical helical springs made of an anisotropic material are solved simultaneously by the transfer matrix method. The overall transfer matrix of the helix is computed up to any desired accuracy by using the effective numerical algorithm available in the literature. The theoretical results are verified with the reported values, which were obtained theoretically and experimentally for straight beams and helical springs. A parametric study is performed to investigate the effects of the number of active coils, the helix pitch angle and material types on the first six natural frequencies of helical springs with circular section and fixed-fixed ends. (C) 2001 Academic Press.