When a child is diagnosed with a chronic life threatening illness there is a significant impact on the entire family. Siblings are at risk for psychological adaptation problems because of their unique relationship with the ill child and the effect of that illness on family functioning. Few studies have been reported which examine the impact of chronic life-threatening illnesses in children on healthy siblings.The purpose of this study was to investigate the predisposing risk and protective factors that affect the psychological adaptation of healthy siblings of a child with Cystic Fibrosis (CF). A descriptive study was conducted using a convenience sample from two CF centers. The relationship between several variables was explored using non- parametric correlations.In this sample, significant negative correlations were found between parental stress and their reported financial well being and emotional or behavioral problems in the well siblings prior to the diagnosis of CF was made in the sick sibling and perceived parental support.For adolescent siblings, The Behavioral Symptoms Index (BSI) was correlated to reported stress in their parents and negatively correlated to the parental perceived support. The Emotional Symptoms Index (ESI) was correlated to the BSI. A significant negative correlation was also noted between age and Internalizing Behaviors. These relationships were not significant for the child siblings in this sample.There were no significant relationships among gender, maternal education, and caretaker for the well sibling when the child with CF was hospitalized and their Internalizing and Externalizing Behaviors. The health care teams in these clinics seldom discussed CF with the well sibling. However, for the children in this study, there was a significant negative correlation with this discussion and their Externalizing Behaviors.Findings from this study suggest that the family environment, especially parental stress and perceived social support, may affect the adaptation of the well sibling. Adolescent siblings were more at risk for this environmental influence than their younger counterparts