To determine the effects of central nervous system (CNS) involvement in systemic Lupus erythematosus (SLE) on cerebrospinal fluid (CSF), CSF studies of 35 cases of SLE (22 cases with CNS involvement and 13 cases without CNS involvement) were analyzed and compared with 20 normal CSF studies. The mean values of several CSF parameters were significantly changed in the group of SLE with CNS involvement (increased protein, white blood cell (WBC) counts and pressure, and decreased glucose contents), but not in the group of SLE without CNS involvement. In 22 case of SLE with CNS involvement, 10 cases (45.5%) were more than 4 WBC's in CSF, 12 cases (54.6%) more than 40 mg/dl of CSF protein and 8 cases (36.4%) less than 40 mg/dl of CSF glucose. CSF pressure was increased more than 200 mm CSF in 41.2%. On the other hand, in 13 cases of SLE without CNS involvement, the CSF studies were almost normal except 4 cases (30.8%) of more than 40 mg/dl in CSF protein.