Background: Racing thoughts, crowded thoughts and flight of ideas are frequent symptoms in mood disorders, but the underlying subjective experience of overactivation of thought processes remains poorly documented. Methods: Qualitative analysis of audiotaped interviews explored subjective experience of thought overactivation in patients with mood disorders (sample 1, n = 45). Quantitative analysis considered the properties of a newly developed rating scale in sample 1, in an additional sample of patients with mood disorders (sample 2, n = 37) and in healthy subjects (sample 3, n = 38). Results: Qualitative analysis of individual interviews revealed that 5 conceptual categories characterized thought overactivation: sequential thought flow, overstimulation, competition for resource allocation, unexpected/unexplained onset, and association with mood and emotions. A principal component analysis of the initial 16-item rating scale indicated that a single component explained 55.9% of the variance, with major and exclusive contributions from 9 items, which were retained in the final 9-item Subjective Thought Overactivation Questionnaire (STOQ; Cronbach's α = 0.95). Total score correlated significantly with activation, depression and perceived conflict subscales of the Internal State Scale (ISS; rs = 0.57-0.66, p < 0.001). It was associated with decreased well-being (ISS; rs = -0.48, p = 0.001) and increased state anxiety (State-Trait Anxiety Inventory; rs = 0.60, p < 0.001). The STOQ score was significantly higher in patients than in healthy subjects. It allowed distinguishing between ISS mood states, with the highest median score in mixed states. Limitations: Sample size, representativeness, possible bias in qualitative analysis, and quality of expert consensus. Conclusions: Qualitative analysis of clinical interviews, together with a new short rating scale, contributed to a documentation of subjective thought overactivation, an important but often undetected feature in mood disorders. © 2013 S. Karger AG, Basel.