This is a protocol for a Cochrane Review (Prognosis). The objectives are as follows:

The primary objective of this review is to synthesise the available evidence on the proportion of individuals who have a diagnosis of autism spectrum disorder at baseline and at follow‐up one or more years later.

The secondary objectives of this review are to:

1. investigate whether there are differences in the proportions of individuals with autism spectrum disorder who maintain a diagnosis at follow‐up dependent on use of the different classification systems (i.e. DSM or ICD criteria) and their revisions; and

2. investigate the proportion of individuals with autism spectrum disorder who maintain diagnosis at follow‐up in important subgroups of individuals, including those of different ages and those with different language levels (verbal/non‐verbal; standard score ≤ 70 or > 70), IQs (≤ 70 or > 70), adaptive behaviour (standard score ≤ 70 or > 70), and different diagnostic subgroups (Asperger's syndrome/disorder, autistic disorder, childhood autism, PDD‐NOS, atypical autism, PDD and autism spectrum disorder).

We will investigate potential sources of heterogeneity that may impact outcomes such as differences in study participation, study design, length of follow‐up, participant attrition and participant outcome measurement factors. We will use internationally recognised standards for systematic reviews to guide the review.