A multisite collaborative study assessed the frequency and intensity of emotional/behavioral distress in siblings of children with cancer. A sample of 254 siblings, aged 4 to 18 years, and their parents completed interviews and self-report measures 6 to 42 (average 22.5) months after diagnosis of cancer in a brother or sister. Matched controls were obtained from respondents to the Child Health Supplement of the National Health Interview Survey administered in 1988 (CHS88). Before diagnosis, the prevalence of parent-reported emotional/behavioral problems among siblings was similar to that in the general population (7.7% vs 6.3%; p = not significant). After diagnosis, prevalence rose to 18% among siblings. When siblings were grouped according to the presence or absence of problems exacerbated by and/or arising after diagnosis, four levels of adaptation, consistent with scores on the Behavior Problem Scales from the CHS88, emerged. This differentiation may help explain inconsistencies in sibling response reported previously and provides a framework for investigating factors that enhance adaptation.