Cognitive‐behavioural models have linked unacceptable or repugnant thoughts in obsessive–compulsive disorder (OCD) with vulnerable self‐themes and fear‐of‐self concerns. To investigate this notion, Aardema and coworkers recently developed and validated the Fear of Self‐Questionnaire (FSQ) in non‐clinical samples, finding it had strong internal inconsistency, and good divergent and convergent validity, including strong relationships to obsessional symptoms and with other processes implicated in cognitive models of OCD (e.g., obsessive beliefs and inferential confusion). The current article describes two studies that aim to replicate and extend these findings in clinical OCD and non‐clinical samples. Study 1 investigated the psychometric properties of an Italian translation of the FSQ in a non‐clinical sample (n = 405). Results of confirmatory factor analysis supported the unidimensionality of the scale; the FSQ also showed very good internal consistency and temporal stability. Study 2 investigated the role of fear of self in OCD symptoms, and unacceptable thoughts and repugnant obsessions in particular, using a clinical OCD sample (n = 76). As expected, fear of self was a unique, major predictor of unacceptable thoughts independent of negative mood states and obsessive beliefs. Moreover, even when considered with obsessive beliefs, anxiety and depression, the feared self was the only unique predictor of obsessionality, providing support for the notion that self‐themes could explain why some intrusions convert into obsessions, whereas others do not. Implications for current cognitive‐behavioural models are discussed. Copyright © 2015 John Wiley & Sons, Ltd. Key Practitioner Message Unacceptable thoughts in OCD have been linked with vulnerable self‐themes and a fear of self. Aardema and coworkers recently developed and validated the Fear of Self‐Questionnaire (FSQ). Study 1 investigated the Psychometric properties of an Italian translation of the FSQ in a non‐clinical sample. Study 2 investigated the role of fear of self in unacceptable thoughts, using a clinical OCD sample. Fear of self was a unique, major predictor of unacceptable thoughts in OCD over and beyond obsessive beliefs.