Anti-ribosomal P (anti-P) antibodies (IgG, IgA, IgM) were measured by an enzyme-linked immunosorbent assay (ELISA) in serial serum samples from the patients with systemic lupus erythematosus (SLE), and these clinical significance was investigated. Neuropsychiatric manifestations were observed in 24 of 144 patients with SLE. Twelve patients had lupus psychosis, nine convulsion or coma and three primary schizophrenia. In the group with lupus psychosis, serum C3 was significantly higher (p < 0.05) and anti-ds-DNA antibody was significantly lower (p < 0.05) than in the other patients. The titer of anti-P antibodies was significantly higher in the group with lupus psychosis than that of the other patients. In a longitudinal study, IgA and IgM classes of anti-P antibody were elevated at the onset of psychosis. The titer decreased following a remission of psychosis. Therefore, the measurement of immunoglobulin classes of anti-P antibody will be useful for the diagnosis and follow-up of patients with lupus psychosis.