The current American College of Rheumatology (ACR) classification criteria for systemic lupus erythematosus (SLE) have proven to be inadequate in the current research climate. In a time of great enthusiasm for clinical trials in SLE, both sponsors and investigators have raised concerns about the current criteria, which were officially last revised in 1983 (with minor modifications, never validated, in 1989). The current criteria over-represent cutaneous lupus, with 4 criteria (whereas every other organ receives only 1). The current criteria need to be updated to represent advances in serologic testing (e.g., complement components, new autoantibodies), improved diagnostic and laboratory testing (e.g., renal biopsy findings, urine protein to creatinine ratios) and broader definitions of organ involvement (e.g., redefining neurologic involvement as more than "seizures" or "psychosis"). The Systemic Lupus International Collaborating Clinics (SLICC) has set a standard for cooperation and productivity in the development and validation of clinical trial measures for SLE, including the SLE Disease Activity Index (SLDEAI), the SLICC Damage Index (later also adopted by the ACR), and the SF-36. SLICC consists of nearly 30 academic centers worldwide. In this study, SLICC will: 1) identify, by frequency analysis, the feasibility of proposed classification criteria; 2) sequentially enroll SLE patients and those with related diseases into a systematic protocol to test proposed classification criteria using traditional and classification tree methodology; and 3) participate in a validation exercise of the generated classification criteria in a new set of patients. This project will lead to updated, validated SLE classification criteria that can be immediately used by both industry and academic clinical trials.