Background: There is not yet a pathognomonic laboratory test for the diagnosis of Behcet’s Disease (BD). The diagnosis is therefore based on clinical manifestations. Mucocutaneous lesions are the most important and the most frequent clinical manifestations of BD. Objective: To study the mucocutaneous lesions systematically. In a great number of patients. Some of these clinical pictures are poorly defined in the literature, and some of them are still unknown. Methods: From 1992 to 1998 we screened systematically all patients attending the Behcet’s Disease Research Unit (BDRU), Rheumatology Research Center, Tehran University for Medical Sciences, for mucocutaneous lesions. Only recurrent lesions were taken in consideration. A total of 3751 patients were seen and registered in our database. The mean number of new patients seen every year, during the last 10 years, was 302 patients per year. Results: Besides the classical lesions, we observed unusual and rare forms of mucous membrane, cutaneous, and subcutaneous lesions. Mucous Membrane lesions: 1) Oral aphthosis is the most important lesion of BD (96.1%±0.6). They are not different from other aphthosis, but they are often smaller. There are also the Punctiform, the Miliaria, and the Herpetiform aphthosis. 2) Genital aphthosis was seen in 63.9%±1.5 of patients. In females they are often larger, deeper, and very painful. Giant forms are seen sometimes. In male they are seen more often on the scrotum. Recurrent conjunctival aphthosis is rare. It is small and ephemera. 3) Ulceration and erosions are different from aphthous lesions. They can take multiple and various shapes without a specific characteristic. 4) Purpura and erythema may also be seen on the mucous membrane. Skin Lesions: Is the second most frequent manifestation of BD. It was seen in 67.4%±1.5 of the patients. The pathergy phenomenon, which is a skin hyper reactivity to trauma, was seen frequently in BD. The most important cutaneous lesions of BD is the pseudo folliculitis. It is a small erythematous lesion, surmounted in the center by a round and non-acuminated pustule, which is dome shaped, sterile, and without hair in the center. They are situated mainly on the lower limbs. Other lesions are 1) Small round erythemato-edematous lesions. 2) Cutaneous aphthosis, which is the most characteristic lesion of BD. It is a round and punched out ulceration with a white yellowish necrosis on the bottom. It leaves a scar after healing. 3) Small nodules. 4) Behcet’s Cellulitis; it is a large painful erythematous lesion. It is not infectious nor a superficial thrombophlebitis. In the literature, it was mistakenly taken as Sweet syndrome. The biopsy in all lesions shows a vasculitis, leucocytoclastic or lymphocytic. Subcutaneous Lesions: Erythema nodosum is seen in 22.8%±1.3 of cases. It is a frequent and relapsing lesion. It is a subcutaneous node, painful, red mauve, with different sizes, and often surrounded with an erythematous and edematous ring. Other lesions are rarely seen 1) Subcutaneous lesions, like Erythema Induratum of Bazin. 2) Suppurative panniculitis. Conclusion: Our study showed a wide variety of muco-cutaneous lesions. The lesions are not specific of BD, but they have their own characteristic. Those characteristics, along with the coexistence of different forms, can help to diagnose BD.