Over the past decade, children with systemic juvenile idiopathic arthritis have increasingly been found to have a life-threatening lung disease (SJIA-LD). However, the causes and best treatments for this deadly complication are unknown. This project will create a multicenter network focused on understanding this disorder, called the CARRA Registry Research Network for SJIA-LD (CARE-NETS). CARE-NETS will identify a cohort of at least 40 children with SJIA-LD for enrollment in the CARRA Registry. We will develop short data forms to collect clinical information on lung disease, and collect biosamples over time. Finally, we will use these biosamples to perform immune cell phenotyping studies on SJIA-LD patients. Together, this pilot study will launch the first research network that will collect clinical data and serial biosamples from children with SJIA-LD through the CARRA Registry. CARE-NETS is an essential first step to further focused research through CARRA to find the cause of SJIA-LD.