Behcet's disease (BD) has rarely been reported in association with malignant diseases. In most cases the autoimmune nature of the disease itself or immunosuppressive drug use has been blamed for malignant transformation. We report 13 cases of BD concurrent with neoplastic disease as well as treatment-related morbidities in this particular patient group. Between 1986 and 1999, 400 patients were diagnosed as having BD in Hacettepe University Hospitals. Of these 13 patients, 3.25% developed malignant diseases within a median follow-up time of 9.8 years. Solid tumors were diagnosed in 10 patients and haematological or lymphoid malignancies in three. Surgery was performed in seven patients, whereas radiotherapy was applied in six and chemotherapy in eight. A literature review revealed 27 cases of BD associated with malignancies, mostly lymphoid or haematological. Ten of our cases were solid tumors, and to our knowledge most of these are the first reported cases of specific malignancies concurrent with BD. Treatment-related morbidities were wound infection as surgical morbidity in one patient (1/7) and radiotherapy-related morbidity in three (3/6) patients in a median follow-up time of 2 years. Solid tumors in addition to lymphoid and haematological malignancies are also seen during the course of BD. Radiation therapy may cause severe late toxicities in the presence of BD. Chemotherapy and surgery are fairly safe for the treatment of malignancies in BD patients.