In two siblings with systemic lupus erythematosus (SLE), who experienced two episodes of psychosis each, a longitudinal study of autoantibodies, including antibodies to ribosomal P proteins, is described. In two of three evaluable periods of 15 weeks antedating psychosis a rise followed by a spontaneous drop in anti-P levels was recorded. In the third period antibodies to ribosomal protein P were absent. It is concluded that results with single samples are not informative, and that frequent measurement of antibodies to ribosomal protein P in patients with SLE may have limited predictive value for psychosis.