To determine maternal stress and child variables predicting maternal stress, 104 mothers of children with autism spectrum disorder (ASD) and intellectual disability (ID) completed the Dutch version of the Parental Stress Index (PSI; De Brock, Vermulst, Gerris, & Abidin, 1992) every six months over a period of two years. The level of maternal stress remained stable over time. Child characteristics predicting maternal stress are behavioral inflexibility toward objects and initiating social interactions. However, these factors do not predict maternal stress when analyzed in combination with children's emotional and behavioral problems measured on the Child Behavior Checklist (CBCL; Achenbach & Rescorla, 2000). The subscales emotionally reactive behavior, withdrawn behavior and attention problems explain a third of the variance in maternal stress. This study revealed no relation between maternal stress and children's developmental age and IQ, receptive and expressive language, adaptive behavior, severity and subtype of ASD, behavioral flexibility toward the environment and persons, initiating and responding to joint attention, initiating and responding to behavioral requests, responding to social interactions and the other subscales of the CBCL. Findings are discussed in relation to the clinical and non-clinical norm groups of the PSI, the limitations of the study and clinical practice. (Contains 5 tables and 1 figure.)