Reaction to a child's diagnosis can strongly affect the parent, but little is known about the aspects of the diagnostic process associated with such reactions. The pre-diagnostic levels of anxiety, depression, and parenting stress of 67 volunteer mothers of children subsequently diagnosed with autism spectrum disorder were assessed, along with the children's functioning. The speed and number of professionals involved in the diagnosis were recorded, and mothers completed a post-diagnosis assessment of their perceptions of the process. Longer actual and perceived diagnosis speeds predicted resolved reactions to diagnosis. Mothers' psychological states pre-diagnosis did not predict reactions to diagnosis. Providing a measured diagnostic process, and strong relationship with mothers, rather than aiming for speed alone, may well be more productive for the parent and child.