Primitive high-alumina olivine tholeiites have been documented from a wide area of Cenozoic volcanism in the northwestern United States. Several new localities extend the geographic range of these rocks to the Cascade Range of northern Oregon, the Picture Gorge region of north-central Oregon, and the Powder River volcanic field of northeastern Oregon. These new sites also extend the age range over which these primitive magmas were generated in the northwestern United States to between 16 Ma and the present. All of the high-alumina olivine tholeiites are of remarkably uniform chemical and mineralogical composition and are similar to primitive ocean-ridge and island-arc tholeiites. The wide distribution in time and space of this magma type and uniform primitive composition suggest that a relatively homogeneous oceanic mantle source underlies much of the northwestern United States. Basalts from the different volcanic provinces exhibit varied evolutionary trends away from this parental composition, reflecting the distinct magmatic processes involved in each province. Trace element and rare earth element abundances in the primitive high-alumina olivine tholeiite lavas record the involvement of crustal material in their genesis. It is argued that this chemical signature was acquired by assimilation of crustal material by a primitive magma such as mid-ocean ridge basalt (MORB) en route to the surface.