The influence of polydispersity on the dynamic structure factor of macromolecules in dilute solution is investigated. Analytical expressions are derived for the translational diffusion coefficient. It is found that the translational diffusion coefficient is less influenced by polydispersity than the mean square radius of gyration. Moreover, the shape of the dynamic structure factor at various scattering vectors is investigated. For a polydisperse macromolecular solution the dynamic structure factor is not a single-exponential function at small scattering vectors. The first cumulant of the dynamic structure factor is compared with dynamic light scattering measurements on polydisperse polysaccharides in dilute solution. Our theoretical calculations agree well with experimental data.