The psychological well-being of mothers raising a child with a developmental disability varies with the nature of the disability. Most research, however, has been focused on Down syndrome and autism. We added mothers whose adolescent or young adult son or daughter has fragile X syndrome. The sample was comprised of mothers of a child with fragile X syndrome (n = 22), Down syndrome (n = 39), or autism (n = 174). Mothers of individuals with fragile X syndrome displayed lower levels of well-being than those of individuals with Down syndrome, but higher levels than mothers of individuals with autism, although group differences varied somewhat across different dimensions of well-being. The most consistent predictor of maternal outcomes was the adolescent or young adult's behavioral symptoms.