The purpose of this study was to assess whether premorbid signs, such as thought disorder, could predict the subsequent manifestation of psychiatric disorders. A group of 75 adoptees at high genetic risk for schizophrenia and 96 low-risk adoptees without any psychiatric disorder at the initial assessment were assessed blindly with the Thought Disorder Index (TDI). Their psychiatric status was re-assessed according to DSM-III-R criteria in a re-interview 11 years later and based on available registers 16 years later. High scores on several TDI variables at the initial assessment predicted a psychiatric disorder of all adoptees at follow-up. Prediction was statistically unsuccessful among the high-risk adoptees because of the small number of cases, but high scores at the 0.50 severity level did predict mental disorders among the low-risk adoptees.