Early old age and the period around retirement are associated with a widening in socioeconomic inequalities in health. There are few studies that address the stress-biological factors related to this widening. This study examined whether retirement is associated with more advantageous (steeper) diurnal cortisol profiles, and differences in this association by occupational grade. Data from the 7th (2002-2004), 8th (2006), and 9th (2007-09) phases of the London-based Whitehall II civil servants study were analysed. Thousand hundred and forty three respondents who were employed at phase 8 (mean age 59.9 years) and who had salivary cortisol measured from five samples collected across the day at phases 7 and 9 were analysed. Retirement was associated with steeper diurnal slopes compared to those who remained in work. Employees in the lowest grades had flatter diurnal cortisol slopes compared to those in the highest grades. Low-grade retirees in particular had flatter diurnal slopes compared to high-grade retirees. Socioeconomic differences in a biomarker associated with stress increase, rather than decrease, around the retirement period. These biological differences associated with transitions into retirement for different occupational groups may partly explain the pattern of widening social inequalities in health in early old age.