Compositions and methods, including vaccines and pharmaceutical compositions for inducing or enhancing an immune response are disclosed based on the discovery of useful immunological adjuvant properties in a synthetic, glucopyranosyl lipid adjuvant (GLA) that is provided in substantially homogeneous form. Chemically defined, synthetic GLA offers a consistent vaccine component from lot to lot without the fluctuations in contaminants or activity that compromise natural-product adjuvants. Also provided are vaccines and pharmaceutical compositions that include GLA and one or more of an antigen, a Toll-like receptor (TLR) agonist, a co-adjuvant and a carrier such as a pharmaceutical carrier.