Cognitive models of Obsessive-Compulsive Disorder [OCD; e.g., Rachman, 1997, Behaviour Research and Therapy, 35, 793–802; Salkovskis, 1985, Behaviour Research and Therapy, 23, 571–583] propose that negative interpretations of intrusive thoughts and images are central to the development and maintenance of OCD. Despite consistent findings that specific interpretations (e.g., heightened responsibility) contribute to the maintenance of OC symptoms [see Salkovskis & Forrester, 2002, Theory, assessment, and treatment, New York: Pergamon], no research has examined the role of these interpretations in the development of OCD or whether OCD-related beliefs predict changes in symptom levels. In the current study, 377 undergraduate students participated in a 6-week prospective study for which they completed measures of OCD-related beliefs, life events, and OC symptoms. OCD-related beliefs and negative life events each significantly predicted residual change in OC symptoms over 6-weeks. However, there was not support for a cognitive–vulnerability–stress interaction. Exploratory analyses suggest that OCD-related beliefs may be more predictive of some particular types of OCD symptoms (e.g., neutralizing, ordering, and obsessing) than others (e.g., hoarding and washing). Findings from this study are important for clarifying predictors of changes in OCD symptom levels and for stimulating research on vulnerability to OCD.