Autoantibodies to anti-ribosomal P protein have been recognized in patients with systemic lupus erythematosus (SLE) in widely variable proportions of unselected patients. Presence of anti-ribosomal P antibodies was retrospectively studied in 69 patients with SLE during disease exacerbations and remissions or during continuously active disease. Anti-ribosomal P antibodies were positive in 21/69 patients during active disease, with an overall prevalence of 30.4%. Prevalence in patients with active nephritis was 75.0% (15/20), P value by Fisher's exact test of 8.39 x 10(-7). In 12/13 patients (92.3%), anti-P disappeared during periods of disease remission, P = 0.0002. In 17/21 patients (81.0%) with anti-ribosomal P antibodies, anti-dsDNA antibodies were also positive. In 47 patients without anti-P, 23/47 (48.9%) were also positive for anti-dsDNA. In 9/12 patients (75.0%) titers of anti-dsDNA antibodies correlated with anti-P during disease exacerbations and remissions, P = 0.004. The higher prevalence of anti-P in patients with lupus nephritis with disappearance during disease remissions supports the hypothesis of an immunopathogenetic role of these antibodies in lupus nephritis. There was also a strong association between anti-dsDNA and anti-P antibodies.