There is considerable evidence for a unitary and dimensional view of the genetic vulnerability to symptoms of anxiety and depression. The GENESiS (Genetic Environmental-Nature of Emotional States in Siblings) Study aims to use a multivariate approach to detect genetic loci that contribute to individual differences in this vulnerability dimension. The study used the UK General Practice Research Framework to generate a community-based sample of siblings. Questionnaire measures of anxiety/depression included the short form of the neuroticism scale from the revised Eysenck Personality Questionnaire (EPQ-N), the General Health Questionnaire (GHQ-12), and the anxious arousal and high positive affect subscales from the Mood and Anxiety Symptoms Questionnaire (MASQ-AA and MASQ-HPA). Genetic model-fitting of 2658 unselected sibships provided evidence for a single common genetic (familial) factor that accounted for a substantial proportion of the genetic variances and covariances of these four measures. Using the parameter estimates of this model, we constructed a composite index of this common genetic factor. This index, which has a sib correlation of 0.22, will be used as a quantitative phenotype in the molecular genetic phase of GENESiS.