An analysis is presented of the correlation between electron-dislocation scattering and electron-impurity scattering and its effect on the specific dislocation resistivity rho D/ND of metals. It is shown that, in contrast to the usual assumption, rho D/ND is not independent of the dislocation density ND. For very large values of ND, of the order of 109 cm-2 for pure samples, rho D/ND is indeed independent of ND. However, for lower values of ND, of the order of 107-108 cm-2, the calculation yields that rho D/ND depends on ND, varying approximately as C1-C 2lg(ND/NDth) where N Dth is a threshold value. The variation of rho D/ND with ND is found to be about an order of magnitude for aluminium and for the noble metals. An experiment is proposed for measuring this predicted dependence of rho D/ND on N D. The few available experimental data for the ND dependence of rho D/ND for Cu and Be are found to be in close agreement with the present theory.