There is no way of predicting whether a patient with recurrent oral ulcerations (ROU) will develop Behçet's disease (BD). In the absence of a valid laboratory test to exclude BD, such oral ulcerations result in a diagnostic problem when they occur as the sole and earliest manifestation of disease. We assessed the prognosis of ROU by performing prospective evaluations of 67 patients who had only a history of ROU and were registered at the Behçet's Disease Specialty Clinic at Severance Hospital of Yonsei University, Seoul, Korea. Thirty-five patients (52.2%) developed overt manifestations of BD at an average of 7.7 years after the onset of ROU. The frequency of recurrence was 9.8 times per year in progressive cases. From these results, it appears that highly recurrent ROU is a warning signal for BD. Careful examinations of patients, including their minor symptoms, additional laboratory tests, and regular follow-ups by physicians are required for proper diagnosis.