Family physicians' experience of using an online evidence retrieval system in routine clinical work was measured. Participants' assessments regarding the usefulness of the online evidence to answer their clinical questions were determined using three feedback channels: voluntary online feedback during the trial; randomized prompted feedback during a specific online evidence search; and responses in a post-trial survey. Using an analysis of the web logs, search outcome was related to patterns of searching in terms of number of searches and documents viewed. The majority of clinicians' searches were related to patient care and information retrieved was rated as useful in answering these questions. On average clinicians conducted two searches and viewed one document per clinical question. Search sessions which resulted in "completely answering" a clinical question involved significantly fewer searches than less successful searches. The three feedback channels provided different information but consistent overall results. The findings indicate that clinicians' assessments of online evidence use are linked to search patterns and online feedback is useful for measuring the effectiveness of systems.