This paper proposes an analytical approach to capturing gradual institutional change in mature welfare states. The approach takes account of both public and occupational provisions of income security and investigates differences between diverse groups in the labour market, various income levels within such groups, as well as changes over time. The analytical approach is then applied to the empirical case of Swedish sickness insurance, on which new data covering the last 30 years have been collected. The results show that sickness insurance as a whole still offers generous replacement rates but that there has been an institutional shift in the provision of income protection. The earlier system of universal public provision that was closely related to former income has developed into a system where this is the case only for certain groups in the labour market while the rest are dependent on collectively negotiated solutions. Accordingly the traditional Swedish institutional model in which an all-encompassing public system promotes equality of status has been transformed over time. In the absence of an adequate analytical approach this is a fact that has not been fully recognised in earlier welfare state research.