<p>This paper examines the relationship between the cross-sectional and lifetime income distributions using a simple model of relative income mobility. It asks whether cross-sectional comparisons between countries can provide a good indication of lifetime inequality differences if income mobility is similar, and whether lifetime inequality increases by less than cross-sectional inequality if the latter increases as a result of higher mobility. Analytical and simulation methods are used to show that the answer to both questions is negative. Comparisons must allow for different types of mobility, the nature of the age-income profile and the age distribution in each country.</p>