Investigating the seasonal asymmetry of violent behaviour has a long history. Despite this, there still remains considerable debate about the nature and aetiology of this phenomenon. Reports on homicide, for example, are mixed: some have found homicide seasonality but most have not. In contrast, all published studies on assault report that this behaviour is seasonal. Moreover, only two studies, both using US data, have examined the seasonal variation of assault and homicide in the same population over the same period of time. One group found assault was seasonal but homicide was not, whilst the other found, overall, that both homicide and assault were seasonal. This first of these findings seems paradoxical, in that there is no seasonal variation in injury related deaths (i.e. homicides), despite the antecedent behaviour (i.e. assaults) having a seasonal pattern of occurrence. We examined the seasonal variation in homicide and assault in UK and found a similar result. Furthermore, our findings are not easily understandable using conventional social models of seasonal behaviour and we suggest biologically mediated seasonal variation in the capacity of equally injured individuals to survive trauma may also play a role, which should be investigated further.