This study investigated the relationship between repetitive behaviors and sensory behavior to the parenting stress of mothers of boys with fragile X syndrome and mothers of boys with autism. Participants consisted of two groups: 51 mothers with boys diagnosed with fragile X syndrome (M= 71.3, SD= 56.5) and 30 mothers with boys diagnosed with autism (M= 86.8, SD= 29.3). Data was taken from an extant data base of two completed studies. Mothers of the participants completed the Parenting Stress Index-SF, the Repetitive Behavior Scale- R, and the Sensory Experiences Questionnaire. Results indicated that parenting stress was predicted by group, repetitive behavior, and sensory behavior which accounted for 41% of the variance. Group was a main effect with the mothers of boys with autism having higher parenting stress and repetitive behavior was a main effect predicting parenting stress for both groups. In subsequent analyses run separately for each group, sensory behavior was predictive of parenting stress in the fragile X group but not in the autism group. Determining how repetitive behavior and sensory behavior contribute to parenting stress for mothers of boys with fragile X syndrome and mothers of boys with autism adds to the literature. It also provides information for the family, teachers, and outside professionals that can be used to develop interventions and in turn reduce parental stress levels.