The course of psychoses of schizophrenic type follows rules which are still not adequately understood. It is, however, clear that certain symptoms appear mostly early, others only late. With this hypothesis in mind, we studied 44 final phase patients whose main symptom was disordered thinking of the schizophasic type and whose illness was of at least 10 years' standing. The most important finding of this study is that the varied and unspecific initial phase progresses into a highly specific syndrome. The symptoms initially registered include various disorders of thinking in less than one third of patients. In no case did these involve schizophasia. In a second phase symptoms were observed such as paralogism, echolalia, verbigeration, circumstantiality, neologism, hypotonic thinking, perseveration, blocking. The symptoms of schizophasia are only recognizable in a third phase and are highly specific. This enables us to confute the claim that psychiatric syndromes are not clinically specific. The three phases described above also provide evidence for the biological nature of this endogenous psychosis.