This investigation evaluates the frequency of various subtypes of thought, language, and communication disorders in 113 patients with diagnoses of mania, depression, and schizophrenia. It indicates that some types of thought disorder considered important occur so infrequently as to be of little diagnostic value, such as neologisms or blocking. The traditional concept of thought disorder, which emphasizes associative loosening, is also of little value, since associative loosening occurs frequently in mania as well as in schizophrenia. This investigation demonstrates that associative loosening can no longer be considered pathognomonic of schizophrenia. On the other hand, an approach that defines various subtypes of thought disorder and uses a concept of negative-vs-positive thought disorder does often permit a distinction between mania and schizophrenia. It is recommended that the practice of referring globally to "thought disorder," as if it were homogeneous, be avoided in the future and instead that the specific subtypes occurring in particular patients be noted in both clinical practice and research.