The effect of daily work stress on the next morning's awakening cortisol level was determined in a sample of 124 mothers (M age = 49.89, SD = 6.33) of adolescents and adults with developmental disabilities and compared to 115 mothers (M age = 46.19, SD = 7.08) of individuals without disabilities. Mothers participated in 8 days of diary telephone interviews and provided saliva samples. Multilevel models revealed that mothers of individuals with developmental disabilities had lower awakening cortisol levels than comparison mothers. Work stress interacted with parental status to predict the awakening cortisol level on the following morning. When mothers of individuals with developmental disabilities experienced a work stressor, their awakening cortisol level was significantly higher on the subsequent morning, but for comparison mothers, work stressors were not significantly associated with cortisol level. Findings extend understanding of the differential impacts of specific types of stressors on physiological functioning of mothers of individuals with and without developmental disabilities.