Patient aggression in psychiatry is a prominent problem for all concerned. In this prospective survey, we recorded and analyzed all violent incidents involving patients in a cantonal psychiatric hospital in Switzerland in order to determine the characteristics of aggression. The Staff Observation Aggression Scale Revised (SOAS-R) was used. During the study period, 815 patients were admitted to the hospital. 71 (63.4%) of the 110 violent incidents were perpetrated by male patients. The majority of aggressive incidents were perpetrated in the vicinity of the patients’ rooms, and were triggered mainly by refusals to adhere to the patient’s demands, or by patients’ use of toxic substances. The most frequent type of aggression was verbal, and the principal target was nursing personnel, who felt threatened in most of the cases. To terminate the aggression, the predominant measures were communication with the patient and coercive measures. This study clearly demonstrates that aggression concerns psychiatric nursing personnel, and recommendations reinforcing security and predictive measures in order to improve the management of aggression.