Lataster T, Collip D, Lardinois M, van Os J, Myin-Germeys I. Evidence for a familial correlation between increased reactivity to stress and positive psychotic symptoms. Objective: This study tested the hypothesis that stress-reactivity may represent an intermediary phenotype underlying positive psychotic symptoms. It was examined whether: (i) stress-reactivity clusters within families of psychotic patients and (ii) stress-reactivity in relatives cosegregates with positive symptoms in patients. Method: The sample consisted of 40 patients and 47 siblings of these patients. The Experience Sampling Method (ESM – a structured diary technique) was used to measure stress-reactivity. Positive symptoms in patients were measured with the Comprehensive Assessment of Symptoms and History. Results: Within-trait, cross-sib associations showed a significant association between stress-reactivity in the patient and stress-reactivity in their siblings. Significant cross-trait, cross-sib associations were established showing a significant association between positive psychotic symptoms in the patient and stress-reactivity in the sibling. Conclusion: The findings show familial clustering of increased stress-reactivity, suggesting common aetiological influences, probably both genetic and environmental, underlying stress-reactivity in the siblings and patients. In addition, the results underscore the hypothesis that increased stress-reactivity is an unconfounded mechanism of risk underlying the positive symptoms of psychotic disorders.