Objective Sjögren's syndrome ( SS ) and systemic lupus erythematosus ( SLE ) are related by clinical and serologic manifestations as well as genetic risks. Both diseases are more commonly found in women than in men, at a ratio of ~10 to 1. Common X chromosome aneuploidies, 47, XXY and 47, XXX , are enriched among men and women, respectively, in either disease, suggesting a dose effect on the X chromosome. Methods We examined cohorts of SS and SLE patients by constructing intensity plots of X chromosome single‐nucleotide polymorphism alleles, along with determining the karyotype of selected patients. Results Among ~2,500 women with SLE , we found 3 patients with a triple mosaic, consisting of 45,X/46, XX /47, XXX . Among ~2,100 women with SS , 1 patient had 45,X/46, XX /47, XXX , with a triplication of the distal p arm of the X chromosome in the 47, XXX cells. Neither the triple mosaic nor the partial triplication was found among the controls. In another SS cohort, we found a mother/daughter pair with partial triplication of this same region of the X chromosome. The triple mosaic occurs in ~1 in 25,000–50,000 live female births, while partial triplications are even rarer. Conclusion Very rare X chromosome abnormalities are present among patients with either SS or SLE and may inform the location of a gene(s) that mediates an X dose effect, as well as critical cell types in which such an effect is operative.