It is well known that individuals from more advantaged social classes enjoy better mental andphysical health than do individuals within lower classes. Various mechanisms have been evoked toexplain the association between socioeconomic status (SES) and health. One mechanism that hasreceived particular attention in recent years is stress. It has been shown that individuals lower inSES report greater exposure to stressful life events and a greater impact of these events on theirlife than individuals higher in SES. In order to measure whether the development of therelationship between SES and mental health is sustained by exposure to high levels ofglucocorticoids, we measured morning salivary cortisol levels as well as cognitive function(memory, attention, and language) in 307 children (from 6 to 16 years of age) from low versushigh SES in the Montreal area in Canada. The results revealed that low SES children from 6 to 10years old present significantly higher salivary cortisol levels when compared to children from highSES. This difference disappears at the time of school transition, and no SES differences areobserved in salivary cortisol levels during high school. However, children from low and high SESdo not differ with regard to memory or to attentional and linguistic functions. Also, mothers oflow SES children reported higher feelings of depression and more unhealthy behaviors, whilemothers of high SES children reported higher stress related to work or family transitions.Altogether, these results show that low SES in young children is related to increased cortisolsecretion, although the impact of SES on cortisol secretion is absent after transition to highschool. These data are interpreted within the context of the equalization process of classpatterning. Four social explanatory factors are suggested to explain the disappearance of SESdifferences in basal cortisol levels after school transition, taking into account the influence offamily environment on the child's secretion of stress hormones.