In a prospective study of 54 patients with acute psychiatric disorders, elevated absolute concentrations of immunoglobulin G (IgG), IgM and complement factor C3 were found in the cerebrospinal fluid (CSF) in seven (P less than 0.001), eight (P less than 0.01) and in four patients, respectively, and in serum in seven, 19 and 17 patients (P less than 0.001 in all), respectively, quantified by automated immunoprecipitin nephelometry in unconcentrated CSF and serum and compared with neurological controls with peripheral neurological symptoms. Elevation of the IgG-index was observed in seven patients (P less than 0.001) and CSF/S albumin ratio in four patients (P less than 0.05). The presence of oligoclonal IgG bands was found in 22 patients (P less than 0.001). The results suggest that viral infections, inflammatory and autoimmune processes may have significance in the etiopathogenesis of acute psychiatric disorders.