The HIV-1 Rev protein facilitates the export of incompletely spliced and unspliced viral mRNAs from the nucleus. Rev polymerizes into two types of filaments in vitro. In the presence of RNA, Rev forms poorly ordered structures, while in the absence of RNA it polymerizes into regular hollow filaments. We have determined the helical structure of the latter filaments by analysis of cryo-electron micrographs, taking into account STEM measurements of mass-per-unit-length. They are made up of Rev dimers, arranged in a six-start helix, with 31 dimers in 2 turns, a pitch angle of 45 degrees, and an interstrand spacing of 3.8 nm. Three-dimensional reconstruction at 2.1 nm resolution reveals a smooth outer surface and a featured inner surface, with outer and inner diameters of similar to 14.8 and similar to 10.4 nm, respectively. The Rev dimer has a "top-hat" shape with a cylinder similar to 3.2 nm in diameter and similar to 2.2 nm high, pointing inward: the thinner rim areas pack together to form the filament wall. Raman spectroscopy shows polymerized Rev to have similar to 54% alpha-helix and 20-24% beta-sheet content, Electron microdiffraction of aligned filaments reveals a broad meridional reflection at similar to (0.51 nm)(-1), suggesting approximate alignment of the alpha-helices with the filament axis. Based on these data, a molecular model for the Rev filament is proposed.