Gambling is acknowledged to have many negative effects on the individual. However, from a psychological perspective, aggression as an outcome of gambling has been overlooked. This paper investigates the dynamics of the relationship between aggression and slot machine gambling. A non-participation observation study observed a small group of aggressive individuals (n = 8; identified in a previous study) over a period of eight weeks. Four categories of aggressive behaviour were confirmed from previous research (verbal aggression towards the gambling arcade staff; verbal aggression towards the slot machines; verbal aggression towards other slot machine players; and physical aggression towards the slot ma-chines). From the in-depth observations, possible reasons motivating these types of aggressive behaviour are discussed. It is suggested that the frustration, guilt and embarrassment of losing are the prime causes of such aggression.