The human immunodeficiency virus type 1 Vpu protein enhances virus particle release from infected cells, decreases the tendency of syncytia formation and induces degradation of human CD4 receptor. It is known that the cytoplasmic part of Vpu is responsible for direct interaction to and degradation of CD4. The tertiary fold of the Vpu cytoplasmic domain in aqueous solution was determined employing NMR spectroscopy and molecular-dynamics simulated-annealing protocols. We found a very well defined amphipathic alpha-helix in the membrane proximal part (40-50), a less well defined helix (60-68), and a short alpha-helix at the C-terminus (75-79). We further determined the overall tertiary structure based on long-range nuclear Overhauser enhancement effects. Correlation of results from mutation experiments of Vpu and the structure data is discussed.