This prospective study aimed to examine whether illness-related negative emotions mediate the relationship of cognitive reappraisal and expressive suppression to the well-being of 99 patients with rheumatoid arthritis or multiple sclerosis. After adjusting for disease and patient-related parameters, only cognitive reappraisal was associated with physical and psychological well-being through emotions. Expressive suppression was associated with psychological well-being only for patients reporting less use of cognitive reappraisal. These results underscore the need for prospective studies that will investigate the long-term impact of emotion regulation on adaptation to chronic illness and the conditions under which this impact takes place.