Objective: To investigate factors influencing variations in clinicians' use of an online evidence retrieval system. Setting: Public hospitals in New South Wales, Australia. Method: Web log analysis demonstrated considerable variation in rates of evidence use by clinicians at different hospitals. Focus groups and interviews were held with 61 staff from three hospitals, two with high rates of use and one with a low rate of use, to explore variation in evidence use. Results: Differences between hospitals' and professional groups' (doctors, nurses and allied health) use of online evidence could be explained by organizational, professional and cultural factors. These included the presence of champions, organizational cultures which supported evidence-based practice (EBP), and database searching skills of individual clinicians. Staff shortages, ease of access and time taken to use the online evidence system were cited as barriers to use at the low use site, but no objective differences in these measures were found between the high and low use sites. Conclusion: Social and cultural factors were found to be better discriminators of high and low evidence use than technical factors.