Psychological adjustment to having one's child diagnosed with an autism spectrum disorder has important implications for a parent's mental health. In a longitudinal study, we examined the association between maternal adjustment to the diagnosis and measures of distress and well-being in 90 mothers of children with autism (baseline and 18 months). We used a novel 30-item scale "Adjustment to the Diagnosis of Autism." Factor analysis identified three dimensions of adjustment: acceptance, self-blame, and despair. Acceptance appeared to be a protective response, as it was associated with lower depressive symptoms, cross-sectionally and over time. Conversely, caregivers with increasing levels of self-blame and despair about the diagnosis over 18 months had worsening of mental health and satisfaction with life during this period.