We collected information about the costs of three interventions (dissemination of full guidelines or prioritised review criteria or criteria plus feedback) to improve care of adults with stable angina or asthma within the context of a randomised controlled trial. The cost of criteria with feedback was pound180 per practice compared to pound9.60-31.45 for the other interventions. Total mean costs borne by practices varied between pound838 and pound2127 per practice depending upon disease and intervention type.