The relationship of temperament, atypical behaviors, and adaptive behavior of young boys with Fragile X syndrome on mothers' parenting stress was analyzed. Twenty-six boys with Fragile X syndrome (30-88 months of age) participated. The overall development of the participants was significantly delayed with a specific profile of adaptive behaviors (i.e., strengths in domestic daily living) and behavioral challenges (i.e., high levels of detachment and hypersensitive or hyperactive behavior). Approximately 90% of the mothers reported very high levels of parenting stress. Correlational and regression analyses revealed that temperament and atypical behavior contributed significantly to the level of parenting stress. Implications for early intervention services for families with a child with Fragile X syndrome were discussed. (Contains 4 tables and 1 footnote.)