The term conflict of interests is applied to those situations in which the research validity and integrity may be influenced by a secondary interest, typically an economic benefit, but also an ideological, personal or professional interest. In this work we describe some ways of conflict of interests--particularly those related with the publication of clinical and epidemiological research supported by the industry--and the regulation of this problem from medical journals, including references to the situation in Spain. The conflict of interest is not synonymous of scientific fraud neither malpraxis in research, but in the medical literature there exists enough evidence to consider it as an important source of biases. The usual form of facing the conflict of interests is to make it public, so that readers can judge its importance. The editorial policies of the Spanish journals are, in general, far from giving importance to this problem, an aspect which could favor an attitude of the investigators, to maintain funding or to obtain new contracts, unnecessarily subordinated to the interests of the companies.