This study explores the relationship between disease type and disease activity, and the psychological status of siblings of chronically ill children. Closest-age siblings of children with Crohn's disease (CD) (n = 41) and ulcerative colitis (UC) (n = 24) were assessed for psychological disorder and psychological style, using reliable and standardized measures. The disease activity of the patients was also evaluated. Assessment occurred during outpatient visits to a pediatric gastroenterology division. Results indicated that CD siblings had more psychological disorder than UC siblings. This was not due to greater acute disease activity in the CD patients. However, as a group, siblings of the sickest CD patients displayed more “internalizing” behaviors, whereas siblings of the healthiest CD patients displayed more “externalizing” behaviors. UC siblings, who were psychologically healthier, displayed “externalizing” behaviors regardless of the patient's disease activity. These results are in accord with our previous findings on family functioning, which, together, yield a heuristic model representing the different patterns of biopsychosocial interaction for CD in contrast to UC. J Dev Behav Pediatr 9:66–72, 1988. Index terms: sibling, psychological status, chronic illness, biopsychosocial.