In order to detect diagnostic clues to identify patients with chronic immune thrombocytopenia which is likely to develop systemic lupus erythematosus (SLE), a retrospective study was conducted. Of 39 patients with chronic immune thrombocytopenia, 4 fulfilled the diagnostic criteria being between 4.5 and 14 years after the initial diagnosis of "idiopathic" thrombocytopenic purpura. The remaining 35 patients were found to be idiopathic thrombocytopenic purpura (ITP) after at least a 5-year follow-up. It was noted that in patients with SLE who had thrombocytopenia at presentation, only 4 or 5 findings of diagnostic criteria were observed. Of particular interest, all patients who developed SLE were positive for anti-SS-A antibody, in contrast none of the patients with ITP had this antibody. Furthermore, despite the few clinical manifestations, we noted lupus peritonitis in 1 patient and pulmonary hypertension in another patient. In conclusion, the anti-SS-A antibody is an excellent clue for the early detection of SLE occurring as "idiopathic" thrombocytopenic purpura.