['Variations in the treatment of multiple sclerosis (MS) occur. This study was performed to obtain insight into the reasons why patients with MS use or do not use immunomodulating treatment (DMT).', 'We held a survey on the use of DMT in a group of 1572 patients with MS.', 'Response rate was 89%. Of the patients, 31% were never on DMT, 21% had stopped DMT, 30% were still on the first DMT, and 18% had changed. The treatment-naive patients were significantly older, had a longer duration of disease, and had a lower number of relapses per year (P < 0.0001). The most important reasons not to start the treatment of MS were the severity and phase of the disease (too mild or PP), possible side effects, the advice of the neurologist, and uncertainty about the effect of DMT. Fifteen percent of the patients stated that when choosing treatment, the information definitely had not been sufficient to make a proper choice. Reasons to stop or switch DMT were mainly side effects (primarily influenza-like symptoms or injection site reactions), especially in RR-MS, and the uncertainty of action or failure of DMT, especially in SP-MS. According to the opinion of the patients, the neurologist had the most decisive role in starting or changing DMT, whilst the patient was primarily responsible in deciding to stop DMT.'] Do reasons patients give to use or not to use immunomodulating agents for multiple sclerosis?