Aims: Flexible-learning first aid courses are increasingly common due to reduced classroom contact time. This study compared retention of first aid knowledge and basic life support (BLS) skills three months after a two-day, classroom-based first aid course (STD) to one utilizing on-line theory learning at home followed by one day of classroom training (FLEX). Methods: In this prospective randomized controlled trial, 256 participants with internet access and no first aid related training for at least five years were randomly allocated to a STD or FLEX course. Assessment was conducted immediately after training and again three months later. Each participant was allocated a theory and a BLS score, which were summed and averaged to create an equally-weighted "combined score" of first aid knowledge and skills. Results: There was no significant difference in theory scores between the STD and FLEX groups immediately after training and after three months. STD participants had significantly higher BLS scores immediately after training (p = 0.001) and three months later (p = 0.046). Males had significantly higher BLS scores after training (p less than 0.001), but not three months later (p = 0.02). Participants older than 46 years had significantly lower BLS scores than younger participants (p less than 0.001). There was no significant difference in combined scores between the STD and FLEX groups or between genders, education or age groups either immediately after training or three months later. Conclusion: After replacing one day of classroom-based training with on-line theory training, there was no significant difference in the first aid competencies of the study population, as measured by an equally-weighted combined score of basic life support and first aid theory. (Contains 3 tables and 1 figure.)