Epub ahead of print Aim Maturity-onset diabetes of the young is a monogenic form of familial, young-onset diabetes. It is rare (∼1% diabetes) and may be misdiagnosed as Type 1 diabetes and inappropriately treated with insulin. Type 1 diabetes is characterized by the presence of islet autoantibodies, including glutamate decarboxylase (GAD) and islet antigen-2 (IA-2) antibodies. The prevalence of islet autoantibodies is unknown in maturity-onset diabetes of the young and may have the potential to differentiate this form of diabetes from Type 1 diabetes. The aim of this study was to determine the prevalence of GAD and IA-2 antibodies in patients with maturity-onset diabetes of the young and Type 1 diabetes. Methods We measured plasma GAD and IA-2 antibodies in 508 patients with the most common forms of maturity-onset diabetes of the young (GCK: n = 227; HNF1A: n = 229; HNF4A: n = 52) and 98 patients with newly diagnosed Type 1 diabetes (diagnosed <6 months). Autoantibodies were considered positive if ≥ 99th centile of 500 adult control subjects. Results GAD and/or IA-2 antibodies were present in 80/98 (82%) patients with Type 1 diabetes and 5/508 (1%) patients with maturity-onset diabetes of the young. In the cohort with Type 1 diabetes, both GAD and IA-2 antibodies were detected in 37.8% of patients, GAD only in 24.5% and IA-2 only in 19.4%. All five patients with maturity-onset diabetes of the young with detectable antibodies had GAD antibodies and none had detectable IA-2 antibodies. Trials carried out in primary care typically involve complex interventions that require considerable planning if they are to be implemented successfully. The role of the statistician in promoting both robust study design and appropriate statistical analysis is an important contribution to a multi-disciplinary primary care research group. Issues in the design of complex interventions have been addressed in the Medical Research Council's new guidance document and over the past 7 years by the Royal Statistical Society's Primary Health Care Study Group. With the aim of raising the profile of statistics and building research capability in this area, particularly with respect to methodological issues, the study group meetings have covered a wide range of topics that have been of interest to statisticians and non-statisticians alike. The aim of this article is to provide an overview of the statistical issues that have arisen over the years related to the design and evaluation of trials in primary care, to provide useful examples and references for further study and ultimately to promote good practice in the conduct of complex interventions carried out in primary care and other health care settings. Throughout we have given particular emphasis to statistical issues related to the design of cluster randomised trials.