Background: Early intensive applied behaviour analysis-based interventions are intensive interventions for autistic children that are often delivered on a one-to-one basis for 20-50 hours per week. Objectives: To evaluate the clinical effectiveness and cost-effectiveness of early intensive applied behaviour analysis-based interventions for autistic children, based on current evidence. Methods: A systematic review and individual participant data meta-analysis were conducted to evaluate the clinical effectiveness of an early intensive applied behaviour analysis-based intervention for autistic children. An economic analysis included a review of existing analyses and the development of a new model. Results: Twenty studies were included in the clinical review. Individual participant data were retrieved from 15 of these studies. Results favoured the interventions when assessing adaptive behaviour after 2 years compared with treatment as usual/eclectic interventions (mean difference 7.00, 95% confidence interval 1.95 to 12.06). In analyses of cognitive ability (intelligence quotient), results favoured the interventions by approximately 10 points after 1 year (mean difference 9.16, 95% confidence interval 4.38 to 13.93) and 2 years (mean difference 14.13, 95% confidence interval 9.16 to 19.10). Evidence for other outcomes was limited and meta-analyses were generally inconclusive. There was no evidence that the effect of the interventions varied with characteristics of the children, but data were limited. Adopting a £30,000 per quality-adjusted life-year threshold, the results of the cost-effectiveness analysis indicate that early intensive applied behaviour analysis-based interventions would need to generate larger benefits or cost savings to be cost-effective. Adopting a public sector perspective and making pessimistic assumptions about long-term effects, the incremental cost-effectiveness ratio for early intensive applied behaviour analysis-based therapy compared with treatment as usual is £189,122 per quality-adjusted life-year. When optimistic assumptions are made, the incremental cost-effectiveness ratio is £46,768 per quality-adjusted life-year. Scenario analyses indicated that these interventions can potentially be cost-effective if long-term improvements persist into adulthood, or if they have significant impact on educational placement. Care should be taken when interpreting these scenarios owing to the limited data. Conclusions: This review found limited evidence that early intensive applied behaviour analysis-based interventions may improve cognitive ability and adaptive behaviour, but the long-term impact of the interventions remains unknown. The economic analysis is constrained by the limited effectiveness evidence, but suggests that these interventions are unlikely to be cost-effective unless clear long-term benefits, or a substantial change in which schools children attend, can be identified. Future work: Further studies into the effectiveness of early intensive applied behaviour analysis-based interventions may be warranted if they include well-defined, alternative interventions as comparators and collect relevant outcomes. Consideration should be given to future studies that not only address whether or not early intensive applied behaviour analysis-based interventions are clinically effective, but also aim to identify which components of early intensive applied behaviour analysis-based interventions might drive effectiveness. Study registration: This study is registered as PROSPERO CRD42017068303.