This paper provides an axiomatization of a cardinal welfare measure for the evaluation of income and opportunity. Following Roemer's (1998) formulation of equality of opportunity, we divide the society in n effort groups consisting of subsets of agents who exert a comparable degree of effort. We show that social welfare can be measured as the sum of n partial indices, one for each effort group, each of which consists of a function of the aggregate income of the group and its dispersion (measured by Theil's first inequality index).