A mode-coupling treatment of diffusion in supercritical fluids is presented. Both neat fluids and dilute attractive supercritical solutions are considered. The solute velocity time correlation function and diffusion coefficient are obtained from the mode-coupling theory (MCT) method and from molecular dynamics simulations. Theory is shown to be in good agreement with simulation. The effect of the solute-solvent interaction strength and solvent clustering on the solute diffusion coefficient is analyzed within the MCT framework. Theoretical results for the diffusion coefficient are compared to the experimental data on the self-diffusion in supercritical xenon and the diffusion of the Xe+ ion in Xe. (C) 2003 American Institute of Physics.