We evaluated outcome and the clinical value of cognitive impairment in systemic lupus erythematosus (SLE). Fifty-one consecutive SLE subjects with or without overt nervous system involvement received two comprehensive neuropsychiatric and neuropsychological assessments, including the Mental Deterioration Battery, the Mini Mental State Examination (MMSE), and tests from the Wechsler Adult Intelligence Scale. The two neuropsychological assessments were made when subjects were in stable neurological condition. Twenty-seven patients were found to have neuropsychiatric symptoms (NP-SLE) at the first assessment, and three others developed them during the follow-up. Fifteen patients (10 NP-SLE) had cognitive impairment at the first assessment. At retest the cognitive deficit persisted in all patients but one (non-NP-SLE) and had developed in four others. In the cognitively impaired subjects scores on MMSE approached the cutoff for an overt dementing condition. No progressively decreasing scores were found on any of the tests. No relationships were shown between neuropsychological diagnosis and neuropsychiatric disorder, neuroradiological findings, disease activity, or steroid and nonsteroid immunosuppressive therapy. Cognitive impairment thus seems to be a stable symptom of CNS involvement in SLE. It corresponds to the subjective complaint of intellectual difficulties and marginal performance on the MMSE. Intellectual deterioration may occur in patients without other symptoms of NP-SLE. Standardized neuropsychological testing methods should be used routinely to assess SLE patients.