This study presents a theoretical model of the general stress state found in helical implantable leads in bending. The model accurately predicts stress in large filarity coils made from clad wire. The stress is predicted using flexural rigidity of the coil and is developed to include the effects of coil pitch and the relative amounts and composition of the coil materials. The results is verified using finite element models of coils made from MP35N wire and MP35N clad silver wire. The resulting model will be especially useful for the fatigue analysis and engineering design of implantable leads which are built in this manner.