In articles that discuss the issue of access to scientific information, the current opinion is that for a long time to come, journals will still continue to be published in printed version, in spite of many advantages for the users when these journals are available in electronic media. This line of thinking has evolved after examining problems of different orders, especially related to the characteristics of scientific editing. According to this situation, the issue of journals dispersion remains important, in particular for those journals that are supported exclusively in printed form or those that do not offer complete articles for web site access. The aspects of quality and the dissemination potential of journals need to be addressed also. The run off copies of a periodical gives limited information related to its dispersion. To generate more accurate dimension about the dispersion of periodicals, other additional information is required. Possibly an analysis of the list of subscribers kept by the edi tors, may facilitate such a study. However, the issue that still remains is how to express these different types of information in a single parameter which could be used for comparing the dispersion of different periodicals. A manner to express this dispersion was obtained using mathematical considerations relating the following factors: the number “p” of possessors of the periodical (it may be referred to institutions, libraries or to subscribers), the number “r” of states (it may be towns, regions or countries) where the possessors are located, and the total number “R” of states (towns, regions or countries) whose dispersion will be referred. Accordingly the formulated equation, the dispersion index is represented by    

Based on web site information (the libraries and their state location), available in the National Collective Catalogue of Serial Publications (CCN) of the Brazilian Institute of Scientific an d Technical Information (IBICT), dispersion index values between 0.05 and 0.82 were observed for a group of Brazilian health journals. In this study there is a demonstration about the origin of the above equation, and each of the factors of the index were discussed in regards to their dissemination potential of the published information. The above equation can also be used to express literature dispersion of authors, articles or the dispersion of a specific theme in a considered knowledge area, given to “r”, “p” and “R” appropriate meanings.