Background: Although number, frequency and healing time of oral ulcers and pain are generally used for clinical practice and studies in Behcet's disease (BD) and recurrent aphthous stomatitis (RAS), no standardized activity index is currently present to monitor clinical manifestations associated with oral ulcers. The aim of this study was to develop a standardized composite index (CI) to assess oral ulcer activity in BD and RAS. Methods: In this cross-sectional study, 121 patients with BD and 45 patients with RAS were included. Sixty-five percentage of BD and 68.9% of RAS patients were in active stage during the previous 3 months. The developed CI included the presence of oral ulcers, ulcer-related pain and functional status and was evaluated in patients with both active and inactive disease for content validity. Results: Composite index score was observed to be higher in active patients with RAS (6.94 ± 2.19) compared with active BD patients (6.01 ± 2.04) (P = 0.04). The number of oral ulcers and healing time of oral ulcers were significantly higher in RAS compared with BD (P = 0.018, P = 0.001 respectively). CI score correlated with the number of oral ulcers in both BD and RAS (P = 0.000, P = 0.002 respectively). CI score was '0' for inactive patients without oral ulcer in BD and RAS. Conclusions: The presented CI as an oral ulcer activity index seems to be a reliable and suitable tool for evaluating the clinical impact and disease-specific problems in BD and RAS. © 2009 John Wiley & Sons A/S.