In a prospective study, an abnormal white cell count and/or elevated protein concentration in the cerebrospinal fluid (CSF) was observed in 14/54 patients (26%) with acute psychiatric disorders, but in none of the 46 control subjects (p < 0.001). In addition, electrophoretic analysis of immunoglobulin G (IgG) showed an oligoclonal pattern in the CSF of 22 psychiatric patients (41 %). No such pattern could be detected in the control group (p < 0.001). These findings reinforce the value of lumbar puncture and CSF analysis in acute psychiatric disorders.