The objective of this study was to identify reliable and valid instruments to measure cognitive impairment in systemic lupus erythematosus (SLE), and to define minimally important change of cognitive impairment in SLE for clinical trials. Neurocognitive measures used in randomized clinical trials in SLE were reviewed, and response criteria were developed using consensus expert opinion. The definition of cognitive impairment in the ACR nomenclature for neuropsychiatric lupus syndrome was adopted. Cognitive impairment is a deficit of 2.0 or more standard deviations (SD) below the mean, compared to normative data, in the key domains of attention, memory and psychomotor speed. Cognitive decline is defined as a deficit of 1.5-1.9 SD below the mean. Focal decline is defined if impairment exists in one or more measures within one domain, and multifocal decline if impairment exists on measures spanning two or more domains. The combination of ACR neuropsychological battery and the Cognitive Symptoms Inventory (CSI) is recommended to quantitate cognitive function. A clinically important response is defined as an improvement of ≥ 1.0 SD with an effect size of 1.0 in the key domains of the ACR neuropsychological testing, and an improvement of ≥ 1.0 SD with an effect size of 1.0 in functional performance of the CSI. © 2007 SAGE Publications.