Short message service (SMS) has become an integral part of people's lives. It is one of the most popular services for mobile communications. This paper examines the determinants of user intention to use SMS. We proposed and empirically tested a model with seven main determinants of user acceptance and two key moderators (i.e., gender and region). Our data were collected via a survey of 471 mobile phone users in China and Hong Kong. The results showed that perceived usefulness, perceived ease of use, perceived enjoyment, and perceived critical mass were the key direct determinants of user acceptance of SMS. Visibility, subjective norm, and perceived cost-effectiveness were found to have indirect influences on user acceptance via the key determinants. In addition, the results showed that gender and region each played an important role in affecting user acceptance, and the significant determinants vary across regions. We discuss implications related to mobile communication services.