This study investigated the prevalence and clinical significance of anti-ribosomal P protein (anti-P) antibodies in patients with systemic sclerosis (SSc). Serum samples from 150 patients with SSc were examined by indirect immunofluorescence. ELISA and immunoblotting. Anti-P antibodies were detected in four (3%) patients with SSc. Three of the four patients showed SSc/SLE (systemic lupus erythematosus) overlap syndrome, but psychiatric disorders were not observed in these patients. By longitudinal immunoblotting analysis one patient, who was initially diagnosed with SSc, later developed anti-P antibodies along clinical manifestations of SLE. Our data suggest that anti-P antibodies are uncommon in SSc and that the presence of anti-P antibodies in patients with SSc indicates an overlap with SLE.