Background: Understanding the natural history of developmental speech and language impairments can support the selection of children whose difficulties are persistent rather than transitory. It can also provide information against which the effectiveness of intervention can be evaluated. However, natural history data are difficult to collect ethically. Furthermore, as soon as an impairment is identified, the behaviour of those around changes, thus creating some level of intervention. Longitudinal cohort studies, where intervention is minimal, or the control arm of randomized trials have provided the best evidence. However, occasional opportunities arise where service waiting lists can provide data about the progress of children who have not received intervention. This natural history study arose within an ethnically diverse, community paediatric speech and language therapy service in the UK where levels of social disadvantage are high. Aims: To identify (1) the characteristics of the children who attended initial assessment and were selected for treatment; (2) the differences between children who did and did not attend reassessment; and (3) the factors associated with outcomes. Methods & Procedures: A cohort of 545 children were referred and assessed as in need of therapy. Due to resource constraints, intervention was not available for an average of 12 months. Children were invited to attend for a reassessment of need. Initial and follow-up assessments were conducted by experienced clinicians using service guidelines and the Therapy Outcomes Measures Impairment Scale (TOM-I). Descriptive and multivariate regression analyses examined child outcomes for changes in communication impairment, demographic factors and length of wait. Outcomes & Results: At initial assessment, 55% of children presented with severe and profound communication impairments. Children offered appointments at clinics in areas of high social disadvantage were less likely to attend reassessment. By reassessment, 54% of children showed spontaneous improvement (mean TOM-I rating change = 0.58). However, 83% were still judged to require therapy. Approximately 20% of children changed their diagnostic category. Age and impairment severity at initial assessment were the best predictors of continuing requirement for input. Conclusions & Implications: Although children do make spontaneous progress post-assessment and without intervention, it is likely that the majority will continue to be assigned case status by a Speech and Language Therapist. However, when evaluating the effectiveness of interventions, clinicians need to factor in the progress that a proportion of the caseload will make spontaneously. Services should be mindful that a lengthy wait may disproportionately impact children who already face health and educational inequalities.