Objective: Autoimmune diseases (AIDs) cluster in families; however, to what extent AIDs co-occur in MS multiplex families with two or more affected individuals is still controversial. The study aimed to evaluate coexisting AIDs in this type of families from the Netherlands.

Methods: A total of 155 MS multiplex families (155 MS probands, 959 first-degree relatives and 212 spouses) were characterized for a history of 11 AIDs by means of a self-administered questionnaire.

Results: In 43.2% of MS multiplex families, at least one AID was present in the first-degree relatives. Overall, the frequency of AIDs was not significantly different between patients with MS (11%), their first-degree family members (11%) and controls (5.2%). After correction for age at inclusion and gender, the odds ratios (OR) for AIDs were not significant for patients with MS (OR = 1.8 [0.77-4.34], P = .17) and first-degree family members (OR = 2.0 [0.98-4.10], P = .06) when both compared to spouses. The frequency of AIDs in mothers did not differ from that in fathers after correction for sex bias (19% vs 8%, P = .51). A presence of AID was more often reported in maternal than paternal second-degree relatives (23% vs 10%, P = .0020).

Conclusions: Although nearly half of the Dutch MS multiplex families reported an AID, no excess of AIDs was present in patients with MS from multiplex families or their first-degree family members compared to the spouses.