© 2019 The Author(s) 2019. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For permissions, please email: email@example.com.Objectives: CYC remains an important treatment option for Behçet's syndrome (BS) patients with life-threatening manifestations. However, adverse events may occur with CYC and this has led to increased use of biologic agents in other vasculitides. We investigated short and long term adverse events associated with CYC use in BS patients. Methods: We conducted a retrospective chart review of all BS patients treated with CYC between 1972 and 2006. Patients were called in and a standard form was used for collecting demographic characteristics, indication for CYC, its cumulative dose and short term adverse events, defined as those causing discontinuation of CYC, hospitalization and/or death, long term adverse events, including infertility and malignancy, and outcome. Results: Of 5790 BS patients, 198 (3.4%) had used at least one dose of CYC. Main indications were vascular or neurological involvement. After a median follow-up of 17 years, 52 (26%) patients had died, 113 (57%) could be contacted, and 33 (17%) were lost to follow-up. Vascular involvement was the leading cause of death (n = 27). Seventeen (9%) patients experienced short term adverse events with haemorrhagic cystitis being the most common. After a median follow-up of 25 years (interquartile range: 15-26 years), 17 malignancies occurred in 15 (8%) patients. Infertility was experienced by 26 (30%) patients. Conclusion: Long term adverse events such as malignancy and infertility were major problems in our BS patients treated with CYC. These results underline the need for safer treatment modalities that are at least as effective as CYC.