Findings have been divergent regarding the direction of basal cortisol dysregulations resulting from stressor exposure, and seem to differ between young people and adults. Accumulated stress exposure has been suggested to be a risk factor for the development of hypocortisolism. This cross-sectional study aims to examine the impact of cumulative adversity, i.e., the number of adversities, on diurnal salivary cortisol levels, including the cortisol awakening response (CAR), in children without psychiatric disorder. The sample consisted of 130 children (mean age 12.8 years), representing one in each twin pair included in the population-based Child and Adolescent Twin Study in Sweden (CATSS). Information about socioeconomic disadvantage, negative life events and potentially traumatic life events were collected by telephone interview and questionnaires, with parents as informants. Salivary cortisol sampling was performed in the home during two school days: at awakening, +30 min post-awakening, and at bedtime. Results showed that the number of adversities was related to the CAR, diurnal decline and +30 min post-awakening cortisol levels. Children with a moderate amount of cumulative adversity displayed high cortisol measures, while those with a high amount (3 or more) of adversities instead showed levels similar to the non-exposed group, yielding an inverse U-pattern of the association between cortisol and adversity. These results indicate that the accumulation of adversity might be an explanation of patterns of basal cortisol up-regulation in children and that those most severely exposed can exhibit an early stage of down-regulation, an issue which should be further examined in longitudinal studies.