AbstractA high proportion of individuals deemed at elevated risk for psychosis will actually never progress to develop the illness. Pharmaceutical intervention may not be necessary in these cases, and may in fact be damaging depending on the invasiveness of the treatment strategy. This highlights the need for biomarkers that are better able to reliably differentiate between at-risk individuals who will subsequently transition to psychosis and those who will not. Low glutathione (GSH) levels have been observed in schizophrenia and in patients with first-episode psychosis. The aim of this study was to determine the predictive value of erythrocyte GSH levels on the transition to psychosis in individuals at risk of developing the illness. Erythrocyte GSH levels were measured in 36 at-risk individuals, 15 of whom had transitioned to psychosis at the 7-year follow-up. Univariate Cox regression analysis showed that transition to psychosis at the 7-year time point was significantly associated with low GSH levels at baseline. The area under the receiving operating characteristic curve was 0.819, indicating that GSH can be considered a good predictor of outcome. Although these results need to be replicated, adding the criterion ‘low erythrocyte GSH’ to the set of criteria used to identify individuals at risk of psychosis may be indicated.