The aim of this study is to find out what kind of impact differences in the customer’s and supplier’s organizations and business cultures has on the success of an ERP project. The work consists of three different researches using the same material: 25 different projects over a period of 22 years were analysed using the information available. 14 of them were selected for closer examination and finally four key persons representing the supplier, as well as four key persons representing the customer, all with long experience, were interviewed. The more detailed examination was conducted by using form with 103 claims concerning business culture, the way to carry out projects and general attitude to colleagues and working. Each claim was replied to twice: first from the viewpoint of one’s own organization and then from the viewpoint of the other party. Both the customer’s and the supplier’s representatives on each project replied to the questionnaire. 35 respondents completed the form concerning the 14 projects selected for the second part. The interviews were conducted either face to face or via e-mail. The results showed clearly that business culture influences the success of an ERP project. The closer the customer and supplier were to each other in some respects and the further in other respects, the better the prognosis was for the project. To ensure the success of the project, the customer and the supplier companies should be of equal size and have the same type of juridical form (e.g. ltd – ltd and plc – plc) and ownership. The supplier should be less conservative than the customer and the supplier’s personnel should value their free time less than the customer’s personnel do. Both should have simple organizations where a worker belongs to one team or department only and has one immediate superior only. Power and responsibilities should go hand in hand, and project managers should have enough power to complete the projects.