Central nervous system involvement in systemic lupus erythematosus (SLE) requires immediate treatment. We report a case in a 30-year-old woman. Clinical features associated asthenia, headache, right nystagmus and coma. A mechanical ventilation was started. The neurologic pattern appeared three months after an initial treatment with pulsed doses of glucocorticoid (500 mg per day for 3 days) and one month after an oral cyclophosphamid regimen (50 mg twice a week). The cerebral involvement was evidenced by MRI and comparative analysis of the antinuclear auto antibodies (ANA) and the complement components in cerebral spinal fluid (CSF), pleural fluid and serum. The MRI slices showed a well-defined meningeal focal lesion. CSF-cell count was normal. CSF-proteins were elevated. ANA were positive, total complement (UI/l) was low, C4 component (g/l) was 0.11, undetectable and 0.25 respectively in plasma, CSF fluid and pleural fluid. The ANA specific pattern was anti-Sm2. We affirmed that specific cerebral injury was present because there were clinical and imaging features and a decrease of the C3 and C4 component in the CSF. The treatment associated in travenous pulsed doses of methylprednisolone (1000 mg per days for 3 days) and cyclophosphamid (500 mg per day for three days). Mechanical ventilation was with drawn one day after the end of the pulse therapy. The diminution of the complement component could help improving cerebral involvement of SLE. More clinical studies are required.