The role of dendritic diameters in maximizing the effectiveness of synaptic inputs was examined in a cell represented as a single cylinder and in a cortical pyramidal cell using mathematical models and computer stimulations. For current input into one end of a cylinder of fixed physical length, the maximum potential at the other end of the cylinder was obtained when the cylinder diameter was chosen so that the electrotonic length, L, of the cylinder was 2.98. For a steady-state or transient synaptic conductance change into the end of a cylinder, the maximum potential at the other end occurred for a value of L less than 2.98; how much less depended on the magnitude of the conductance change. In the model of the reconstructed cortical pyramidal cell, synaptic inputs at proximal, mid-dendritic, and distal locations were most effective for different, particular sets of dendritic diameters. For each synaptic input, there is a set (probably non-unique) of dendritic diameters which maximizes the effectiveness of that input. Paradoxically, a synaptic input at a given physical distance from the soma may produce a larger change in soma potential when it is at a longer electrotonic distance from the soma than at a shorter one. The dendritic diameters determine which inputs are operating at maximal effectiveness. Changes in Rm or Ri or changes in synaptic conductance magnitude or time course may shift the loci of inputs operating at maximal effectiveness. This would change the weighting of synaptic inputs and possibly affect neuronal function.