Background On a population of 10 million sportsmen in the Netherlands, 3.6 million sports injuries are sustained each year. Behaviour is essential in preventing those injuries. An online tailored intervention, http://www.voorkomblessures.nl , provides Dutch amateur athletes with sport-specific and personalised (tailored) advice. Objective To test the effectiveness of the online intervention on (determinants of) behaviour to prevent sports injuries among amateur runners between 18 and 35 years old. Design In a prospective randomised controlled trial participants were randomly assigned to: experimental group 1 (with baseline), experimental group 2 (without baseline) or control-group. Baseline, 30 min follow up and 3 month follow up data were obtained through online questionnaires between September 2009 and February 2010. Setting Real life setting. Participants After selection on age (18–35), skill level (amateur) and familiarity with the intervention (none) 318 of 352 recruited runners were included in the study and participated at baseline and 30 min follow up measurement. 199 runners participated at 3 month follow up. Intervention Participants in both experimental groups had to view the online intervention for 30 min. Respondents in the control group had to read non running related magazines for 30 min. Main outcome measurements The intervention aims to influence taking four preventive behaviours and/or change their determinants. Results At 30 min follow up significant effects of the intervention are found on various determinants: attitude, risk perception, knowledge and intention. At 3 month follow up significant effects are found on the determinants knowledge and risk perception. More importantly significant effects are shown on two injury preventive behaviours. Conclusions This study shows the online intervention http://www.voorkomblessures.nl succeeds in positively influencing the (determinants of) injury preventive behaviours. More research is needed on long term effects and effects on other types of sport.