This work concerns some selected 3-D flow effects on residence time distribution (RTD) in a screw extruder. The Newtonian problem is emphasized in which the cross- and down-channel flow components can be formally discoupled and treated as two parallel 2-D problems : our stream;function-based analysis of the Newtonian cross-channel flow was combined with the existing 2-D solution for the down-channel flow. The RTD was determined by tracing the streamlines generated from both velocity components for the industrially important cases of fully- and partially-filled channels. The effect of the cross-channel flow on the RTD is discussed in connection with the stagnation flows, the down-channel pressure gradient, and the presence of a free surface in a partially-filled channel. The cross-channel pressure distribution is compared to the conventional I-D theory. The non-Newtonian effects were assessed using the commercial software POLYCAD.