To describe the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K), a modification of SLEDAI to reflect persistent, active disease in those descriptors that had previously only considered new or recurrent occurrences, and to validate SLEDAI-2K against the original SLEDAI as a predictor for mortality and as a measure of global disease activity in the clinic. All visits in our cohort of 960 patients were used to correlate SLEDAI-2K against the original SLEDAI, and the whole cohort was used to validate SLEDAI-2K as a predictor of mortality. A subgroup of 212 patients with SLE followed at the Lupus Clinic who had 5 regular visits, 3-6 months apart, in 1991-93 was also included. An uninvolved clinician evaluated each patient record and assigned a clinical activity level. The SLEDAI score was calculated from the database according to both the original and modified definitions. SLEDAI-2K correlated highly (r = 0.97) with SLEDAI. Both methods for SLEDAI scoring predicted mortality equally (p = 0.0001), and described similarly the range of disease activity as recognized by the clinician. SLEDAI-2K, which allows for persistent activity in rash, mucous membranes, alopecia, and proteinuria, is suitable for use in clinical trials and studies of prognosis in SLE.