health psychology report • volume 7(3), 9 original article background Diagnosis of chronic illness, which constitutes a lifethreatening situation such as in Marfan syndrome or disease with periods of remission and severity such as in juvenile idiopathic arthritis (JIA), leads to problems with the functioning of the adolescent and the whole family system. Therefore the aim of the study was to determine the differences in severity of internalizing and externalizing problems and in the perception of the family system between adolescents with Marfan syndrome, with JIA, and without chronic disease, as well as to determine whether characteristics of the family system are associated with externalizing and internalizing behaviors in those groups. participants and procedure The study involved three groups of adolescents: 1) with Marfan syndrome (n = 25), 2) with JIA (n = 29), and 3) without chronic illness (n = 20). We used the adolescent version of the Child Behavior Checklist (CBCL) Youth Self-Report (YSR) and the Family Assessment Scale. results Our results indicated that adolescents with Marfan syndrome have similar scores compared to adolescents without illness on all subscales concerning both internalizing and externalizing behaviors, as well as the family system's characteristics. Compared to adolescents with JIA, adolescents with Marfan syndrome had lower scores on externalization symptoms and Attention Problems and they also perceive their families as less balanced. Additionally, in the perception of adolescents with Marfan syndrome and with JIA the weaker the family bond, the stronger the intensity of externalizing behaviors. conclusions Although in the light of our novel research, adolescents with Marfan syndrome do not present behavioral problems different from healthy peers, it is worth continuing such research in order to verify the results. Adolescents with JIA present more externalizing behaviors than peers without chronic illness, which is consistent with some previous research. Research on families of an adolescent child with Marfan and JIA are quite novel and they should be continued.