OBJECTIVE: Seasonal variations of violence have been the subject of some controversy. Norway, situated between latitudes 58 degrees and 72 degrees N, has considerable seasonal variations of light and provides a good opportunity for studies of seasonality. METHOD: The monthly numbers of police reports of violent incidents in 1991-1997 were obtained for the entire Norwegian population of 4,450,000 inhabitants and separately for each of seven Norwegian cities at different latitudes. RESULTS: A total of 82,537 episodes of violence were recorded. There was a significant variation in violent incidents between months, with a minimum daily frequency of 28.7 in March and a maximum daily frequency of 35.1 in June. The frequency curve had one significant peak in May through June and another significant peak in October through November. The monthly frequency of violence correlated with the absolute value of monthly change in length of day from the previous month. In the seven cities the highest monthly ratio of observed to expected frequencies increased with latitude. With increasing latitude, the months with the largest increase in violence came later both in the spring and in the fall. CONCLUSIONS: There is a distinct pattern of seasonal variation in the frequency of violence that varies systematically with latitude. This pattern resembles the seasonal pattern of some forms of suicide, hospitalization for affective disorders, and mood and activity in the general population.