It is commonly believed that the full moon exerts an influence on violence and aggression in psychiatric settings. The literature to date is contentious. This study used a robust methodology to examine the hypothesis that there was an increased frequency of violent and aggressive behaviour among hospitalised psychiatric clients at the time of the full moon. Prospective data were collected in five inpatient psychiatric settings across the Northern Sydney Area Health Service. Morrison's hierarchy of violence and aggression was used to rate behaviour. Lunar phases were clearly defined and Poisson regression used to examine relationships between lunar phase and violence. Extraneous temporal variation was considered. No significant relationship was found between total violence and aggression or level of violence and aggression and any phase of the moon. Future research could profitably examine the implications of a belief in the lunar effect among health workers in the face of evidence that no relationship exists between violence, aggression and the lunar cycle.