To characterize the initial clinical and laboratory features of patients with systemic onset juvenile rheumatoid arthritis (soJRA) through a Web-based registry. Patients diagnosed with soJRA in the last 15 years at 3 medical centers in Pennsylvania were identified. Data were collected retrospectively using a Web-based interface in compliance with patient privacy standards. Inferential statistics were used to compare features of patients with and without macrophage activation syndrome. We identified 136 patients; 88% of patients presented with arthritis (8% mono-, 45% oligo-, 47% polyarticular). The most common joints involved were the knee (68% of patients with arthritis), wrist (68%), and ankle (57%). The International League of Associations for Rheumatology criteria for systemic juvenile idiopathic arthritis (SJIA) identified only 30% of patients at presentation. We successfully characterized the presenting features of a relatively rare disease, soJRA, through the use of a Web-based registry. Current classification criteria for SJIA may not be particularly sensitive for diagnosis at presentation.