Background. Educational outreach visits, particularly when combined with social marketing, appear to be a promising approach to modifying health professional behaviour, especially prescribing. Results from previous studies have shown a varying effect. Objective. The purpose of the study is to examine the effect of academic detailing as a method of implementing a clinical guideline in general practice. Methods. A cluster randomized, controlled, blinded study was carried out of the effect of an academic detail visit compared with postal distribution of a guideline for prescribing asthma medication. Half the practices in a Danish county with 100 practices were visited once. The outcome measure was routinely collected data from all Danish pharmacies on the sales of asthma medication. Data were collected monthly for 2 years before to 1 year after the intervention. Results. There was no effect on the pattern of prescription of asthma medicines following the visit, neither immediately nor long term. Conclusion. We found no effect of academic detailing as a single intervention.