A very large Danish epidemiological study of 7590 children will estimate the frequency of tics among children aged 10-12 years. The study will also examine different pre- and perinatal factors as possible contributors to the development of tics. Estimates of the prevalence of Tourette Syndrome (TS) vary greatly among different studies. The range has been reported to be anywhere from 1 to 19 per 1000 children. These large differences may be attributed to small sample sizes, different age distributions and variations in sampling and diagnostic assessment. The largest and best designed study so far identified 25 children with TS out of 4479 (6 per 1000). The present study will use an epidemiological robust design and address tics among 7590 children aged 10-12 years. It will therefore be the largest epidemiological study of tics to date. The basic causes of TS and related tic disorders are poorly understood. Genetic factors are implicated, but the inheritance is complex and not yet understood fully. Furthermore, scientific interest in interactions between genetic and environmental factors has increased. At least three sets of interrelated environmental factors in tic disorders have been implicated: adverse pre and perinatal events, acute and chronic psychosocial stressors, and post infectious autoimmune mechanisms. Many previous case-controlled studies on the effect of prenatal environment were weakened by what is called subject recall bias. The present study of a large birth cohort offers data collected prospectively during pregnancy, an approach that will eliminate this bias. We also plan to offer a unique opportunity for studying environmental risk factors during pregnancy and their influence on early brain development in offspring. In the Scandinavian countries, researchers have a long tradition for combining data from epidemiological studies with national medical registries providing a very valuable opportunity not feasible elsewhere. Søren Dalsgaard, Ph.D., University Hospital in Aarthus, Psychiatric Hospital for Children & Adolescents, Risskov, Denmark Award: $75,000 Tourette Association of America Inc. – Research Grant Award 2003-2004