Most people experience involuntary memories frequently in daily life, and these are thought to play important functional roles. Interestingly, people can experience involuntary memories not only of events they have actually experienced, but also of scenarios they have only imagined. However, this latter phenomenon, and the factors that influence which imagined scenarios do or do not later return involuntarily, are relatively unexplored. The current study investigated whether the vividness with which a scenario is imagined affects the likelihood of later involuntary recall. University students (N = 80) listened to brief descriptions of positive scenarios and imagined these happening to them, with half of the sample randomly allocated to hearing extended versions of the scenarios designed to induce more vivid imagery. Participants then recorded involuntary memories of the imagined scenarios during their daily lives in a three-day diary before returning to the lab. Unexpectedly, there was no effect of the experimental manipulation on vividness of the imagined scenarios or on the number of involuntary memories recorded. However, participants who generated more vivid imagery of the scenarios recorded more involuntary memories of them in the diary. Exploratory analyses revealed further predictors of the quantity and quality of involuntary memories experienced. Overall, the results further our understanding of factors that may affect the likelihood of involuntary retrieval of previously-generated positive imagery, with both theoretical and potential clinical implications.