Background: Information retrieval systems have the potential to improve patient care but little is known about the variables which influence clinicians' uptake and use of systems in routine work. Aim: To determine which factors influenced use of an online evidence retrieval system. Design of study: Computer logs and pre- and post-system survey analysis of a 4-week clinical trial of the Quick Clinical online evidence system involving 227 general practitioners across Australia. Results: Online evidence use was not linked to general practice training or clinical experience but female clinicians conducted more searches than their male counterparts (mean use = 14.38 searches, S.D. = 11.68 versus mean use = 8.50 searches, S.D. = 9.99; t = 2.67, d.f. = 157, P = 0.008). Practice characteristics such as hours worked, type and geographic location of clinic were not associated with search activity. Information seeking was also not related to participants' perceived information needs, computer skills, training nor Internet connection speed. Clinicians who reported direct improvements in patient care as a result of system use had significantly higher rates of system use than other users (mean use = 12.55 searches, S.D. = 13.18 versus mean use = 8.15 searches, S.D. = 9.18; t = 2.322, d.f. = 154 P = 0.022). Comparison of participants' views pre- and post- the trial, showed that post-trial clinicians expressed more positive views about searching for information during a consultation (χ2 = 27.40, d.f. = 4, P ≤ 0.001) and a significantly greater number reported seeking information between consultations as a result of having access to an online evidence system in their consulting rooms (χ2 = 9.818, d.f. = 2, P = 0.010). Conclusion: Clinicians' use of an online evidence system was directly related to their reported experiences of improvements in patient care. Post-trial clinicians positively changed their views about having time to search for information and pursued more questions during clinic hours.