Conventionally, the seasonal pattern in a time series is taken as exogenous. However, two recent studies from the US have suggested, the seasonal component in box office revenue may in part be endogenously determined. Films that are expected to do well tend to be released at times of peak demand. Hence, the seasonal pattern may reflect both the underlying seasonal pattern and the market expansion effect of the films; evidence from the US suggests the former dominates the latter. This article investigates the same issue for the UK using a structural time series model. We find that, although the seasonal pattern varies over time, we are unable to reject the hypothesis that the seasonal factors are constant. This is consistent with findings from the US.