The CONSORT statement has been extended to include guidance for reporting cluster randomised trials. Cluster randomised trials involve randomly assigning groups of individuals (such as families or practices) rather than individuals. This approach is used when individual randomisation may lead to contamination of interventions or when group randomisation is the only feasible method. The extension of the CONSORT statement provides additional information to help readers interpret cluster randomised trials accurately. Key elements of the extended CONSORT statement include: the rationale for using a cluster design, how clustering was incorporated into sample size calculations and analysis, the flow of clusters and individuals through the trial, and the level at which interventions were targeted. The statement also includes specific changes to the checklist items and flow diagram to address these issues. Cluster randomised trials are more complex to design and analyse than individually randomised trials. They require more participants to achieve equivalent statistical power and involve more complex analysis due to the correlation between observations within clusters. The intracluster correlation coefficient (ICC) and design effect are important factors in sample size calculations. The quality of reporting of cluster trials has been poor, with many studies failing to account for clustering in their design or analysis. Recent studies have shown that many cluster trials do not appropriately account for clustering in their design or analysis, leading to potential bias in the results. The extended CONSORT statement provides guidance on how to report cluster randomised trials, including specific items for the title and abstract, introduction, methods, participants, intervention, objectives, outcome, sample size, sequence generation, allocation concealment, statistical methods, results, and discussion. The flow diagram is also modified to include the flow of clusters and individuals through the trial. The CONSORT group recommends that journal editors include these guidelines in their instructions to authors to improve the quality of reporting of cluster randomised trials. The guidelines are also available on the CONSORT website.The CONSORT statement has been extended to include guidance for reporting cluster randomised trials. Cluster randomised trials involve randomly assigning groups of individuals (such as families or practices) rather than individuals. This approach is used when individual randomisation may lead to contamination of interventions or when group randomisation is the only feasible method. The extension of the CONSORT statement provides additional information to help readers interpret cluster randomised trials accurately. Key elements of the extended CONSORT statement include: the rationale for using a cluster design, how clustering was incorporated into sample size calculations and analysis, the flow of clusters and individuals through the trial, and the level at which interventions were targeted. The statement also includes specific changes to the checklist items and flow diagram to address these issues. Cluster randomised trials are more complex to design and analyse than individually randomised trials. They require more participants to achieve equivalent statistical power and involve more complex analysis due to the correlation between observations within clusters. The intracluster correlation coefficient (ICC) and design effect are important factors in sample size calculations. The quality of reporting of cluster trials has been poor, with many studies failing to account for clustering in their design or analysis. Recent studies have shown that many cluster trials do not appropriately account for clustering in their design or analysis, leading to potential bias in the results. The extended CONSORT statement provides guidance on how to report cluster randomised trials, including specific items for the title and abstract, introduction, methods, participants, intervention, objectives, outcome, sample size, sequence generation, allocation concealment, statistical methods, results, and discussion. The flow diagram is also modified to include the flow of clusters and individuals through the trial. The CONSORT group recommends that journal editors include these guidelines in their instructions to authors to improve the quality of reporting of cluster randomised trials. The guidelines are also available on the CONSORT website.