Using daily diary methods, mothers of adolescents and adults with ASD (n = 86) were contrasted with a nationally representative comparison group of mothers of similarly-aged unaffected children (n = 171) with respect to the diurnal rhythm of cortisol. Mothers of adolescents and adults with ASD were found to have significantly lower levels of cortisol throughout the day. Within the ASD sample, the son or daughter’s history of behavior problems interacted with daily behavior problems to predict the morning rise of the mother’s cortisol. A history of elevated behavior problems moderated the effect of behavior problems the day before on maternal cortisol level. Implications for interventions for both the mother and the individual with ASD are suggested.