Multiple sclerosis (MS) is a complex disease triggered by environmental and genetic agents, and clinically characterized by bout onset (BOMS) or progressive onset (PrMS). We collected clinical and familial aggregation data in a cohort of 518 Italian PrMS patients, and compared with 400 BOMS cases. An increased prevalence of MS in first-degree relatives of Italian PrMS was found. Familial aggregation is not influenced by probands' clinical course, and there is no disease course concordance within MS families. These data are useful in counseling MS patients affected with different clinical courses of the disease.