Behçet's disease (Adamantiades-Behçet's disease, ABD) is a multisystemic inflammatory disease, the pathogenesis of which is still a mystery. Many questions are still to be answered and the available diverse data need to be brought together to be compared and analysed. There is at least consensus on the effect of possible, but currently unknown, environmental triggering factor(s) against a background of genetic susceptibility. The possible aetiological factors form a broad spectrum, with infectious agents being the most probable ones. Whatever the stimulus is, the target tissue seems to be the small blood vessels, with various consequences of either vasculitis and/or thrombosis in many organ systems. The endothelium seems to be the primary target in this disease; however, it may just be the subject of the bizarre behaviour of the immune system. The diverse existing data could be interpreted in favour of either explanation. A similar confusion exists about the thrombotic tendency in Adamantiades-Behçet's disease, in terms of whether a primary hypercoagulability is present or whether it is secondary to inflammation. Recent interesting immunological data promise a way out of the existing dilemma. These findings will be outlined within the context of possible hypotheses and attention will be paid to further investigations that are needed.