BACKGROUND: Racing thoughts is a frequent symptom in mood disorders, particularly mixed depressive states. This paper aims to summarize our current knowledge about its phenomenology and frequency in the spectrum of mood disorders, and to offer a new theoretical framework. METHODS: We made a selective review of original and review papers in Medline and PsychInfo database using the keywords "racing thoughts", "crowded thoughts" and "depressive mixed state" in conjunction with "mood disorders". RESULTS: In the context of a hypomanic state, "racing thoughts" may appear as a result from an excessive production of thoughts, moving quickly from one to the other, and generating a sense of fluidity and pleasantness. In the context of depression, "racing thoughts" are phenomenologically different and better described as "crowded thoughts": they are not only characterized by too many thoughts occurring at the same time in the field of consciousness, but perceived as unpleasant and induce the feeling that ideas are difficult to catch. DISCUSSION AND CLINICAL RELEVANCE: We suggest that crowded thoughts might result from the mixture of a hypomanic component, with an accelerated production of new thoughts (constituting the main source of this symptom in hypomania), and a depressive component, with a deficit of inhibition of previous thoughts (hence making thoughts crowded rather than truly racing). This distinction could help better identify crowded thoughts, and consequently depressive mixed states, which has important implications for therapeutic management. It might also help to further disentangle the psychobiological processes which contribute to the complexity of mood disorders.