BACKGROUND: Recent studies have suggested that there has been an increase in the number of 'warning letters' issued by the US Food and Drug Administration (FDA) despite the publication of the FDA advertising guidelines. However, limited information is available on the description of warning letters. The objective of this study was to analyse the frequency and content of FDA warning letters in relation to promotional claims and discuss the influence of regulatory and industry constraints on promotion. METHODS: All warning letters published by the FDA between 5 May 1995 and 11 June 2007 were reviewed. Warning letters related to promotional issues were included and analysed. Information related to the identification number, date of the warning letter, FDA division that issued the letter, drug name, manufacturer, specific warning problem, type of promotional material and requested action was extracted. Two independent investigators reviewed and classified each PDF file, any differences were discussed until a consensus was reached. RESULTS: Between May 1995 and June 2007 a total of 8692 warning letters were issued, of which 25% were related to drugs. Of these, 206 warning letters focused on drug promotion and were included in this study: 23% were issued in 2005, 15% in 2004 and 14% in 1998. In total, 47% of the warning letters were issued because of false or misleading unapproved doses and uses, 27% failed to disclose risks, 15% cited misleading promotion, 8% related to misleading labelling and 3% promoted false effectiveness claims. DISCUSSION: There is an important variation in the number of warning letters issued in the last decade, probably because of the increasing number of drugs approved by the FDA, drug withdrawal scandals, and the publication of the FDA and the Pharmaceutical Research and Manufacturers of America (PhRMA) guidelines. CONCLUSION: We found that benefit-related claims, such as unapproved uses or doses of drugs, and failure to disclose risks, are the main causes of FDA issued warning letters for promotional claims related to medications.