Densely packed and ordered cobalt nanowire (NW) arrays with aspect ratios (wire length/diameter) varying between 5 and 250 have been fabricated via electrodeposition into alumina templates. The wire length and diameter were controlled by monitoring the total deposited charge or by adjusting the template pore size, respectively. It is observed from room temperature magnetization curves that the magnetic properties of a given array is largely dependent on the aspect ratio and packing factor. It is shown that behavior of magnetic NW arrays are governed to a large extent by the magnetostatic interactions between NWs and that magnitude of the interaction field increases not only with NW diameter (or packing factor) but also with NW length. (C) 2008 American Institute of Physics.