Objectives: To determine the variation in the level of knowledge of the employees of a construction company, after a continuous educational intervention in medical first aid. Methods: Longitudinal experimental study (educational intervention) in a group of workers in Lima. They were divided randomly and all were given the same basal training; locally validated tests were used to measure the level of knowledge in each phase. P values were obtained to determine the differences between the averages by type of intervention. Results: The mean level of basal theoretical knowledge was 1.3 points (from 10 possible points), which tripled to 4.6 after the first training and finally reduced after months. Statistically significant differences were found between the averages of theoretical notes according to educational interventions received (p=0.035), but the theoretical final averages were disapproving. Conclusions: While there was a difference in the notes according to the type of reinforcements, it was not sufficient to obtain an adequate level of knowledge of the subject; is necessary to know whether the training ensure that workers are properly trained, and that is not enough to train them, you also have to ensure that they retain the knowledge gained.