Cardiac leads are often designed and manufactured in the form of helical coils. This paper presents a complete nonlinear theory and a systematic analysis approach so that various large deformation lead problems can be solved with ease and accuracy. The nonlinear force-strain relationships are first derived through two strain parameters. In the case of small deformation, the nonlinear theory is linearized, direct force-strain relationships are obtained in terms of four spring stiffness constants, and the engineering spring theory is proven to be a special case of the linear theory. Examples are given to illustrate various responses of the cardiac lead, and a series of tests is performed to verify the theory developed. The investigation shows that the nonlinear theory agrees very well with the experiments, and the comparison suggests that the engineering spring theory should be limited in the design of critical elements such as cardiac leads.