Behçet's disease (BD) is a multisystem disease, in which recurrent aphthous stomatitis (RAS) is a universal finding. We studied the expression of RAS in patients with BD, and the correlation between major or minor RAS and systemic expression and severity of the disease. Thirty-five patients with BD were studied, of whom 13 (37%) had major, 21 (60%) had minor and one (3%) had herpetiform RAS. The frequency of major RAS was significantly higher compared with a control group of patients with idiopathic RAS (37% vs 9%, P < 0.05). The BD patients with major RAS had significantly more relapses of oral ulceration in a year, higher numbers of oral ulcers per relapse, and longer duration of aphthous episodes, compared with patients with minor RAS. Oral ulcers also appeared at a significantly younger age in patients with major than with minor RAS. However, the systemic expression of the disease, as well as the disease severity score, were similar in patients with major and minor RAS. The results of this study indicate that major RAS is common in patients with BD, and is associated with a more severe, repeated and prolonged oral disease. Nevertheless, the presence of major RAS in BD does not predict a more severe systemic illness.