Online gaming has become one of the most popular forms of Internet applications and online entertainments today, and has reshaped the ways people communicate and interact. Massively multiplayer online games (MMOGs), being highly challenging, interactive, immersive and persistent, are however often presented as being potentially dangerous of leading gamers to play excessively. Despite a rising concern over this emerging global issue, the theoretical understanding of the excessive use of MMOGs is lacking in the IS literature. Therefore, the main objective of this study is to propose a research model to theoretically explain the development of excessive use of MMOGs. We tested our research model in longitudinal design using an online survey with 602 active MMOGs users. Our results suggested that mood regulation derived from using MMOGs predicts the excessive use. Motivations for playing MMOGs, achievement and immersion, are found important in determining mood regulation. This study provides a theoretical explanation of excessive use of MMOGs, and the results help researchers and practitioners understand the main drivers and mechanisms of the development of excessive use of MMOGs.