Siblings of patients with multiple sclerosis have an increased risk of developing the disease. In this report we describe 3 families with multiple affected members, representing the largest published aggregation of cases in first degree relatives. In the 2 families in which HLA tissue-typing was performed the affected individuals shared part of the haplotype HLA-DR2 (+ HLA-DQW1), BfS (+ C2C), C4A3, C4B1. The implications of these findings for the aetiology of multiple sclerosis are discussed.