Ten international pharmaceutical companies were asked by letter to supply their best evidence in support of marketing claims for seventeen products. Fifteen replies were received. Seven replies cited a total of 67 references: 31 contained relevant original data and only 13 were controlled trials, all of which had serious methodological flaws. There were four reports of changes in advertising claims and one company ceased marketing nikethamide in the third world. Standards of evidence used to justify advertising claims are inadequate.