We discuss ILC measurements for a specific MSSM scenario with CP phases, where the lightest neutralino is a good candidate for dark matter, annihilating efficiently through t-channel exchange of light staus. These prospective (CP-even) ILC measurements are then used to fit the underlying model parameters. A collider prediction of the relic density of the neutralino from this fit gives 0.116<{omega}h{sup 2}<0.19 at 95% C.L. CP-odd observables, while being a direct signal of CP violation, do not help in further constraining {omega}h{sup 2}. The interplay with (in)direct detection of dark matter and with measurements of electric dipole moments is also discussed. Finally we comment on collider measurements at higher energies for refining the prediction of {omega}h{sup 2}.