Aims To identify the main factors predicting delays in diagnosis for Autism Spectrum Disorder (ASD) at three stages in the diagnostic process: wait for first appointment; assessment duration, and total wait for diagnosis. Method Data were gathered from 150 case notes (80 child and 70 adult cases) from 16 diagnosing services across Scotland. Results Having more information pre-assessment was associated with a reduced duration of the diagnostic process for children. This relationship was partially mediated by a reduction in the number of contacts required for diagnosis. In adults, having more factors associated with ASD (increased risk) reduced the wait time from referral to first appointment, but increased the overall duration of the diagnostic process. The latter relationship was partially mediated by an increase in the number of contacts required for diagnosis. Conclusion Within children's services, increasing the amount of relevant information available pre-assessment is likely to reduce total duration of the assessment process by reducing number of contacts required. Having a high risk of ASD as an adult appears to result in being seen more quickly following referral, but also to increase the number of contacts needed and assessment duration. As a result, it increases and total duration overall.