The aim of this study was to investigate whether severe formal thought disorders and mature thinking are stable among adoptees (=187) drawn from the Finnish adoptive family study of schizophrenia. A group of 93 adoptees genetically at high risk (HR) and 94 at low risk (LR) for schizophrenia were assessed blindly and reliably using the Index of Primitive Thought (IPT) and the Index of Integration (IOI). Two assessments of the IPT and the IOI were performed with the mean interval of 11 years. Comparisons of the IPT and the IOI mean scores were conducted both at baseline and at follow-up between adoptees at low and high genetic risk, gender, and psychiatric status. The main result was that the IOI as well as the IPT of the adoptees at the initial assessment predicted the IOI and the IPT estimated at follow-up, thus indicating the stability of severe formal thought disorders and mature thinking over time. The stability of IOI or IPT was not related to genetic risk, gender or psychiatric status or their interactions.