Open AccessThis study analyze why the SES-health gradient increases with ageing. We use Statistics Sweden’s Survey of Living Conditions (the ULF). By comparing various SES and health outcome relationships we explore the age increase in health inequality and distinguish between three types of explanations, namely: i) age increase in the causal SES effect; ii) reversed health effect on SES, and iii) lifecycle variation in the measurement errors in SES. Thus, the study indicates that the age increase in health inequality is primarily caused by a reversed causality going from health to annual income, and the probable mechanism is health affecting the labour supply of the individual. The evidence in our study is not conclusive in that we can prove anything, but all the documented evidence supports this conclusion.