Context: General practitioners are frequently involved in clinical trials sponsored by pharmaceutical companies but the effects of participation on their prescribing patterns have not been evaluated. Objective: To determine how conducting a company-sponsored clinical trial influenced physicians' adherence to international treatment recommendations and their prescribing of the pharmaceutical company's drugs. Design, Setting, and Patients: Observational cohort study in Funen County, Denmark, comparing 10 practices that were conducting a trial on asthma medicine with 165 control (non-trial-conducting) practices. The study population included 5439 patients treated with asthma drugs from the trial-conducting practices and 59 574 patients from the control practices. Practices conducted the trial between April 26, 2001, and October 7, 2002. Main Outcome Measures: Adherence to guidelines measured as use of inhaled corticosteroids among asthma patients. Prevalence of use of the company's drugs and the trial sponsor's share of the total volume of asthma drugs prescribed. Results: The baseline proportion of asthma patients using inhaled corticosteroids was 68.5% in trial-conducting and 69.1% in control practices. Conducting the trial did not influence guideline adherence (odds ratio [OR] after 2 years, 1.00; 95% confidence interval [CI], 0.84-1.19). In trial-conducting practices, the sponsoring company's share of the total prescribed volume of asthma drugs increased compared with control practices (6.7%; 95% CI, 3.0%-11.7%). This could be attributed to a significantly higher preference for the company's inhaled corticosteroids (OR, 1.26; 95% CI, 1.04-1.54) and trends toward increased prescribing of the company's other asthma drugs. Conclusion: Conducting a trial sponsored by a pharmaceutical company had no significant impact on physicians' adherence to international treatment recommendations but increased their use of the trial sponsor's drugs.