Information security has actually been a major challenge to most organizations. Indeed, information security is an ongoing risk management process that covers all of the information that needs to be protected. ISO 17799 offers what companies need in order to better manage information security. The best way to implement this standard is to ease the security management process using a methodology that will define will define guidelines, procedures and tools that will be needed along the way. Hence, this paper proposes a methodology to assist companies in assessing their compliance with BS 7799/ ISO 17799 as well as planning and implementing the actions necessary to become compliant or certified to the standard. The concepts and ideas presented here had been applied in a case study involving the Cetrel S/A - Company of Environmental Protection. For this company, responsible for treatment of industrial residues generated by the Camaçari Petrochemical Complex and adjacent areas, to assure confidentiality and integrity of customers' data is a basic requirement.