The influence of molecular packing on the performance of polymer organic field-effect transistors is illustrated in this work. Both close pi-stacking distance and long-range order are important for achieving high mobilities. By aligning the polymers from solution, long-range order is induced, yielding hole mobilities of up to mu(sat) = 1.4 cm(2) V-1 s(-1) and current on/off ratios I-on/I-off of 10(5).