Background Multiple sclerosis (MS) and other autoimmune diseases might cluster. Our aim
was to estimate the relative risk (RR) of other autoimmune diseases among MS patients and
their first-degree relatives in a population-based cohort study. Methods Using the Danish
Multiple Sclerosis Register, the Danish Hospital Discharge Register, and the Danish Civil
Registration System, we estimated RRs for 42 different autoimmune diseases in a
population-based cohort of 12 403 MS patients and 20 798 of their first-degree relatives …

Sir–We read with interest the study by Nielsen, et al.[1] describing the occurrence of
autoimmune disease in patients with multiple sclerosis (MS) and their first degree relatives.
This large Danish cohort has been the subject of many important contributions [2]. If patients
with MS and their first degree relatives were at an increased risk of other autoimmune
diseases, it would be at the very least conceptually important. However, we do not agree
with this position. The data raises the following points: