BehÇet's disease is a multisystem disorder that affects mainly young adults. Conflicting reports of the effects of pregnancy on the course of BehÇet's disease have been reported. We studied 27 pregnant women with BehÇet's disease using questionnaires, clinical examination, and laboratory tests. The condition of 18 patients worsened (66.7%) and 9 patients (33.3%) improved during pregnancy. In the deteriorated group, clinical exacerbation of BehÇet's disease occurred most commonly during the first trimester (77.8%). They were primarily of the suspected or mucocutaneous type, while in the improved group, the majority of patients were of the incomplete and complete type, or the arthritic and ocular type. Of interest was the fact that all patients in the improved group showed exacerbations related to menstruation or postpartum, which indicated that progesterone may be the major hormone influencing the course of BehÇet's disease.