INTRODUCTION Mental retardation is the most frequent disability and is already quite apparent in infancy. The World Health Organisation (WHO) estimates that it affects approximately 3% of the population in industrialised countries. Among the aetiologies that cause mental retardation it would appear that 30% have a genetic origin, although in recent years the progress made in molecular genetics in relation to new mutations and new genes that can account for mental retardation advances at an incredible pace. It is for this reason that, three years ago, a group of clinicians and researchers, most of whom were working in Spain on fragile X syndrome (the most prevalent of the hereditary causes of mental retardation), decided to set up the GIRMOGEN (Genetic Mental Retardation Research Group). Most of us had noticed how many of the clinical cases that we dealt with went undiagnosed and that the exact prevalence of this disability in Spain was not known either. DEVELOPMENT. GIRMOGEN was funded by the Carlos III Health Institute and is made up of eight groups; a ninth group is responsible for coordinating the work. Its members are all involved in clinical studies or research into mental retardation with a genetic origin, and belong to 15 hospitals and to five universities from a total of 11 different autonomous communities in Spain. Some of its aims include gathering all the data on patients and families in a common database for epidemiological and prevalence studies; distributing genes to be studied in order to search for mutations; creating generally approved work protocols, and training professionals in this field.