Knowledge of mothers about vaccination has been documented to influence the decisions they make when it comes to vaccinating their children. The aim of this study was to measure the prevalence of mothers' knowledge about vaccination side effects and determine any significant relationships between the mothers' demographic characters and their knowledge. Methods: The mothers were also asked about their source of information. The study was carried out at Al-Amyria Family Centre in Cairo and included a total of 385 mothers who were there for their child's vaccination session. Results: The prevalence of mothers who had knowledge about side effects was 78.7%, with 32.5% responding that a healthcare provider was the main source, and 26.5% responding that television was the main source of information. These two sources were the main ones, regardless of the mothers' age or education. Nearly all mothers (98.7%) said they continue giving booster doses to their children even if side effects occur. Conclusion: Programs to enhance the mothers' knowledge and provide more information to them can therefore be developed with healthcare professionals playing a major role.