Background & Objectives:Multiple sclerosis (MS) is one of the most common demyelinating diseases of the central nervous system. The disease occurs with higher frequency among families. This study aimed to investigate the frequency and type of familial MS among patients with definite MS registered in the Isfahan Society for MS. Methods: A cross sectional study was performed on 3911 MS patients in Isfahan. All patients had a diagnosis of definite MS. Demographic characteristics, medical history, signs and symptoms at onset, course of disease, having a relative with MS, degree and type of relationship were recorded. Results: Familial MS was found in 11% of patients, with 57. 7% having a first degree relative with MS. Mean age of patients with familial MS was 36.9±10.4 years, with higher rates among women (female to male ratio 2.6). Highest rate for familial MS was observed in sister-sister relations, and brother-sister relation. Lowest rate was observed in father-son relation. Conclusions: Familial MS is more common among sisters while father-son relationship has the lowest association. Female to male ratio is 2.6 in familial MS which shows higher rates of males relative to general population.